diff --git a/messages/de.json b/messages/de.json
index cdbaa9522..630ce0ac1 100644
--- a/messages/de.json
+++ b/messages/de.json
@@ -6834,6 +6834,34 @@
       "headache": "Kopfschmerzen",
       "bodyAches": "Gliederschmerzen",
       "fatigue": "Erschöpfung"
+    },
+    "correlation": {
+      "title": "Wie es verlief",
+      "learning": "Noch am Lernen — erfasse ein paar weitere Tage, um hier Muster zu sehen.",
+      "preOnsetTitle": "Wie es sich ankündigte",
+      "preOnsetBody": "Werte, die vor dem Beginn von deiner Baseline abwichen.",
+      "nadirTitle": "Was sich änderte",
+      "nadirBody": "Wie weit jeder erfasste Wert von deinem typischen Bereich abwich.",
+      "recoveryGapTitle": "Erholungslücke",
+      "recoveryGapLag": "Deine Werte normalisierten sich {days} Tage, nachdem es dir besser ging.",
+      "recoveryGapLead": "Deine Werte normalisierten sich {days} Tage, bevor du Genesung vermerktest.",
+      "recoveryGapSame": "Deine Werte normalisierten sich am selben Tag, an dem es dir besser ging.",
+      "redFlagTitle": "Genauer ansehen",
+      "redFlagSpo2": "Deine Sauerstoffsättigung blieb mehrere Tage niedrig. Wenn das erneut auftritt, suche ärztliche Hilfe.",
+      "redFlagFever": "Du hattest anhaltend Fieber. Wenn das erneut auftritt, suche ärztliche Hilfe.",
+      "direction": {
+        "above": "über deinem Üblichen",
+        "below": "unter deinem Üblichen"
+      }
+    },
+    "insights": {
+      "title": "Dein Krankheitsverlauf",
+      "countOne": "Du hast in diesem Zeitraum 1 Episode erfasst.",
+      "countOther": "Du hast in diesem Zeitraum {count} Episoden erfasst.",
+      "typicalGap": "Typische Erholungslücke: {days} Tage zwischen Besserung und normalisierten Werten.",
+      "typicalGapLearning": "Noch ein paar abgeschlossene Episoden und hier erscheint eine typische Erholungslücke.",
+      "byMonth": "Wann du Episoden erfasst",
+      "none": "In diesem Zeitraum keine Episoden erfasst."
     }
   },
   "coach": {
diff --git a/messages/en.json b/messages/en.json
index 7ca9be83e..1dd848fbb 100644
--- a/messages/en.json
+++ b/messages/en.json
@@ -6834,6 +6834,34 @@
       "headache": "Headache",
       "bodyAches": "Body aches",
       "fatigue": "Fatigue"
+    },
+    "correlation": {
+      "title": "How it unfolded",
+      "learning": "Still learning — log a few more days to see patterns here.",
+      "preOnsetTitle": "How it announced itself",
+      "preOnsetBody": "Signals that drifted from your baseline before you marked the onset.",
+      "nadirTitle": "What changed",
+      "nadirBody": "How far each tracked signal strayed from your typical range.",
+      "recoveryGapTitle": "Recovery gap",
+      "recoveryGapLag": "Your numbers settled {days} days after you felt better.",
+      "recoveryGapLead": "Your numbers settled {days} days before you marked recovery.",
+      "recoveryGapSame": "Your numbers settled the same day you felt better.",
+      "redFlagTitle": "Worth a closer look",
+      "redFlagSpo2": "Your oxygen saturation stayed low for several days. If this recurs, seek medical care.",
+      "redFlagFever": "You ran a sustained fever. If this recurs, seek medical care.",
+      "direction": {
+        "above": "above your usual",
+        "below": "below your usual"
+      }
+    },
+    "insights": {
+      "title": "Your illness history",
+      "countOne": "You logged 1 episode in this period.",
+      "countOther": "You logged {count} episodes in this period.",
+      "typicalGap": "Typical recovery gap: {days} days between feeling better and your numbers settling.",
+      "typicalGapLearning": "A few more resolved episodes and a typical recovery gap will appear here.",
+      "byMonth": "When you tend to log episodes",
+      "none": "No episodes logged in this period."
     }
   },
   "coach": {
diff --git a/messages/es.json b/messages/es.json
index 4edefab7d..6d909c863 100644
--- a/messages/es.json
+++ b/messages/es.json
@@ -6834,6 +6834,34 @@
       "headache": "Dolor de cabeza",
       "bodyAches": "Dolores musculares",
       "fatigue": "Fatiga"
+    },
+    "correlation": {
+      "title": "Cómo se desarrolló",
+      "learning": "Aún aprendiendo: registra unos días más para ver patrones aquí.",
+      "preOnsetTitle": "Cómo se anunció",
+      "preOnsetBody": "Señales que se alejaron de tu base antes de marcar el inicio.",
+      "nadirTitle": "Qué cambió",
+      "nadirBody": "Cuánto se alejó cada señal registrada de tu rango habitual.",
+      "recoveryGapTitle": "Brecha de recuperación",
+      "recoveryGapLag": "Tus valores se estabilizaron {days} días después de sentirte mejor.",
+      "recoveryGapLead": "Tus valores se estabilizaron {days} días antes de marcar la recuperación.",
+      "recoveryGapSame": "Tus valores se estabilizaron el mismo día en que te sentiste mejor.",
+      "redFlagTitle": "Conviene revisarlo",
+      "redFlagSpo2": "Tu saturación de oxígeno se mantuvo baja varios días. Si se repite, busca atención médica.",
+      "redFlagFever": "Tuviste fiebre sostenida. Si se repite, busca atención médica.",
+      "direction": {
+        "above": "por encima de lo habitual",
+        "below": "por debajo de lo habitual"
+      }
+    },
+    "insights": {
+      "title": "Tu historial de enfermedades",
+      "countOne": "Registraste 1 episodio en este período.",
+      "countOther": "Registraste {count} episodios en este período.",
+      "typicalGap": "Brecha de recuperación típica: {days} días entre sentirte mejor y la estabilización de tus valores.",
+      "typicalGapLearning": "Con unos pocos episodios resueltos más aparecerá aquí una brecha de recuperación típica.",
+      "byMonth": "Cuándo sueles registrar episodios",
+      "none": "Sin episodios registrados en este período."
     }
   },
   "coach": {
diff --git a/messages/fr.json b/messages/fr.json
index e2644cd04..b86158944 100644
--- a/messages/fr.json
+++ b/messages/fr.json
@@ -6834,6 +6834,34 @@
       "headache": "Mal de tête",
       "bodyAches": "Courbatures",
       "fatigue": "Fatigue"
+    },
+    "correlation": {
+      "title": "Comment cela s'est déroulé",
+      "learning": "Encore en apprentissage — enregistre quelques jours de plus pour voir des tendances ici.",
+      "preOnsetTitle": "Comment cela s'est annoncé",
+      "preOnsetBody": "Signaux qui se sont écartés de ta base avant le début que tu as noté.",
+      "nadirTitle": "Ce qui a changé",
+      "nadirBody": "À quel point chaque signal suivi s'est écarté de ta plage habituelle.",
+      "recoveryGapTitle": "Écart de récupération",
+      "recoveryGapLag": "Tes valeurs se sont stabilisées {days} jours après que tu te sois senti mieux.",
+      "recoveryGapLead": "Tes valeurs se sont stabilisées {days} jours avant que tu notes la guérison.",
+      "recoveryGapSame": "Tes valeurs se sont stabilisées le jour même où tu t'es senti mieux.",
+      "redFlagTitle": "À examiner de plus près",
+      "redFlagSpo2": "Ta saturation en oxygène est restée basse plusieurs jours. Si cela se reproduit, consulte un médecin.",
+      "redFlagFever": "Tu as eu une fièvre persistante. Si cela se reproduit, consulte un médecin.",
+      "direction": {
+        "above": "au-dessus de ton habituel",
+        "below": "en dessous de ton habituel"
+      }
+    },
+    "insights": {
+      "title": "Ton historique de maladies",
+      "countOne": "Tu as enregistré 1 épisode sur cette période.",
+      "countOther": "Tu as enregistré {count} épisodes sur cette période.",
+      "typicalGap": "Écart de récupération typique : {days} jours entre le mieux-être et la stabilisation de tes valeurs.",
+      "typicalGapLearning": "Encore quelques épisodes résolus et un écart de récupération typique apparaîtra ici.",
+      "byMonth": "Quand tu enregistres des épisodes",
+      "none": "Aucun épisode enregistré sur cette période."
     }
   },
   "coach": {
diff --git a/messages/it.json b/messages/it.json
index e6c673f46..387f81694 100644
--- a/messages/it.json
+++ b/messages/it.json
@@ -6834,6 +6834,34 @@
       "headache": "Mal di testa",
       "bodyAches": "Dolori muscolari",
       "fatigue": "Affaticamento"
+    },
+    "correlation": {
+      "title": "Come si è svolto",
+      "learning": "Ancora in apprendimento: registra qualche giorno in più per vedere i pattern qui.",
+      "preOnsetTitle": "Come si è annunciato",
+      "preOnsetBody": "Segnali che si sono allontanati dalla tua base prima dell'inizio che hai segnato.",
+      "nadirTitle": "Cosa è cambiato",
+      "nadirBody": "Quanto ogni segnale registrato si è allontanato dal tuo intervallo abituale.",
+      "recoveryGapTitle": "Divario di recupero",
+      "recoveryGapLag": "I tuoi valori si sono stabilizzati {days} giorni dopo esserti sentito meglio.",
+      "recoveryGapLead": "I tuoi valori si sono stabilizzati {days} giorni prima di segnare la guarigione.",
+      "recoveryGapSame": "I tuoi valori si sono stabilizzati lo stesso giorno in cui ti sei sentito meglio.",
+      "redFlagTitle": "Da guardare più da vicino",
+      "redFlagSpo2": "La tua saturazione di ossigeno è rimasta bassa per diversi giorni. Se si ripete, rivolgiti a un medico.",
+      "redFlagFever": "Hai avuto febbre persistente. Se si ripete, rivolgiti a un medico.",
+      "direction": {
+        "above": "sopra il tuo solito",
+        "below": "sotto il tuo solito"
+      }
+    },
+    "insights": {
+      "title": "La tua storia di malattie",
+      "countOne": "Hai registrato 1 episodio in questo periodo.",
+      "countOther": "Hai registrato {count} episodi in questo periodo.",
+      "typicalGap": "Divario di recupero tipico: {days} giorni tra il sentirti meglio e la stabilizzazione dei valori.",
+      "typicalGapLearning": "Ancora qualche episodio risolto e qui apparirà un divario di recupero tipico.",
+      "byMonth": "Quando tendi a registrare episodi",
+      "none": "Nessun episodio registrato in questo periodo."
     }
   },
   "coach": {
diff --git a/messages/pl.json b/messages/pl.json
index 34d735c60..a1b6d1d8c 100644
--- a/messages/pl.json
+++ b/messages/pl.json
@@ -6834,6 +6834,34 @@
       "headache": "Ból głowy",
       "bodyAches": "Bóle ciała",
       "fatigue": "Zmęczenie"
+    },
+    "correlation": {
+      "title": "Jak przebiegało",
+      "learning": "Wciąż się uczę — zapisz jeszcze kilka dni, aby zobaczyć tu wzorce.",
+      "preOnsetTitle": "Jak się zapowiadało",
+      "preOnsetBody": "Sygnały, które odbiegły od Twojej bazy przed oznaczonym początkiem.",
+      "nadirTitle": "Co się zmieniło",
+      "nadirBody": "Jak daleko każdy śledzony sygnał odbiegł od Twojego typowego zakresu.",
+      "recoveryGapTitle": "Luka rekonwalescencji",
+      "recoveryGapLag": "Twoje wartości ustabilizowały się {days} dni po tym, jak poczułeś się lepiej.",
+      "recoveryGapLead": "Twoje wartości ustabilizowały się {days} dni przed oznaczeniem wyzdrowienia.",
+      "recoveryGapSame": "Twoje wartości ustabilizowały się tego samego dnia, gdy poczułeś się lepiej.",
+      "redFlagTitle": "Warto przyjrzeć się bliżej",
+      "redFlagSpo2": "Twoje wysycenie tlenem pozostawało niskie przez kilka dni. Jeśli się powtórzy, zasięgnij pomocy medycznej.",
+      "redFlagFever": "Miałeś utrzymującą się gorączkę. Jeśli się powtórzy, zasięgnij pomocy medycznej.",
+      "direction": {
+        "above": "powyżej Twojej normy",
+        "below": "poniżej Twojej normy"
+      }
+    },
+    "insights": {
+      "title": "Twoja historia chorób",
+      "countOne": "W tym okresie zapisałeś 1 epizod.",
+      "countOther": "W tym okresie zapisałeś {count} epizodów.",
+      "typicalGap": "Typowa luka rekonwalescencji: {days} dni między poprawą a ustabilizowaniem wartości.",
+      "typicalGapLearning": "Jeszcze kilka zakończonych epizodów i pojawi się tu typowa luka rekonwalescencji.",
+      "byMonth": "Kiedy zwykle zapisujesz epizody",
+      "none": "Brak epizodów zapisanych w tym okresie."
     }
   },
   "coach": {
diff --git a/src/app/api/illness/episodes/[id]/correlation/route.ts b/src/app/api/illness/episodes/[id]/correlation/route.ts
new file mode 100644
index 000000000..a315f69ae
--- /dev/null
+++ b/src/app/api/illness/episodes/[id]/correlation/route.ts
@@ -0,0 +1,87 @@
+/**
+ * `GET /api/illness/episodes/{id}/correlation` — the retrospective correlation
+ * findings for one episode: the pre-onset anomaly scan ("how did it announce
+ * itself"), the nadir / what-dropped, and the recovery-gap (physiological
+ * return-to-baseline vs the felt-better marker).
+ *
+ * Server-authoritative + coverage-gated: the response is a `Derived<T>` flat
+ * wire shape (`status: "ok" | "insufficient"`). When the signal is thin the
+ * engine returns `insufficient` with coverage + a reason — the surface renders
+ * "we're still learning", never a fabricated number. The findings are derived
+ * from the user's OWN baseline (median ± MAD), never a population constant, and
+ * the baseline window is contamination-guarded (ends before the pre-onset
+ * lookback). Retrospective ONLY — never a predictor or diagnoser. Born-gated +
+ * owner-scoped.
+ */
+import { NextRequest } from "next/server";
+
+import { prisma } from "@/lib/db";
+import { apiHandler, requireAuth } from "@/lib/api-handler";
+import { annotate } from "@/lib/logging/context";
+import { apiSuccess, apiError } from "@/lib/api-response";
+import { requireIllnessEnabled } from "@/lib/illness/gate";
+import { computeEpisodeCorrelation } from "@/lib/illness/correlation-read";
+import { DEFAULT_TIMEZONE } from "@/lib/mood/date-key";
+
+type RouteParams = { params: Promise<{ id: string }> };
+
+export const GET = apiHandler(
+  async (_request: NextRequest, { params }: RouteParams) => {
+    const { user } = await requireAuth();
+
+    const gate = await requireIllnessEnabled(user.id);
+    if (!gate.enabled) return gate.response;
+
+    const { id } = await params;
+    const episode = await prisma.illnessEpisode.findUnique({
+      where: { id },
+      select: {
+        id: true,
+        userId: true,
+        deletedAt: true,
+        onsetAt: true,
+        resolvedAt: true,
+        lifecycle: true,
+      },
+    });
+    if (!episode || episode.userId !== user.id || episode.deletedAt !== null) {
+      return apiError("Episode not found", 404);
+    }
+
+    const tz = user.timezone ?? DEFAULT_TIMEZONE;
+    const derived = await computeEpisodeCorrelation(
+      user.id,
+      {
+        id: episode.id,
+        onsetAt: episode.onsetAt,
+        resolvedAt: episode.resolvedAt,
+        lifecycle: episode.lifecycle,
+      },
+      tz,
+    );
+
+    annotate({
+      action: {
+        name: "illness.correlation.read",
+        entity_type: "illness_episode",
+        entity_id: id,
+      },
+      meta: {
+        status: derived.status,
+        recovery_gap_days:
+          derived.status === "ok" ? derived.value.recoveryGapDays : null,
+        red_flags: derived.status === "ok" ? derived.value.redFlags.length : 0,
+      },
+    });
+
+    return apiSuccess({
+      episodeId: id,
+      status: derived.status,
+      value: derived.status === "ok" ? derived.value : null,
+      coverage: derived.coverage,
+      confidence: derived.status === "ok" ? derived.confidence : null,
+      provenance: derived.provenance,
+      reason: derived.status === "insufficient" ? derived.reason : null,
+    });
+  },
+);
diff --git a/src/app/api/illness/insights/route.ts b/src/app/api/illness/insights/route.ts
new file mode 100644
index 000000000..48960abdd
--- /dev/null
+++ b/src/app/api/illness/insights/route.ts
@@ -0,0 +1,107 @@
+/**
+ * `GET /api/illness/insights?windowDays=365` — the cross-episode retrospective
+ * summary: "sick N times this year · typical recovery gap X days", with a
+ * recurrence-by-month tally and a per-type breakdown.
+ *
+ * RETROSPECTIVE ONLY — it summarises past episodes. It is NEVER a predictor or
+ * diagnoser; the recurrence figure is a count, not a forecast. Server-
+ * authoritative + gated: the "typical gap" is withheld below the min-sample
+ * floor (asserts nothing thin). Per-episode gaps come from the same coverage-
+ * gated correlation engine the per-episode route uses, so a thin episode
+ * contributes no gap rather than a wrong one. Born-gated + owner-scoped.
+ */
+import { NextRequest } from "next/server";
+
+import { prisma } from "@/lib/db";
+import { apiHandler, requireAuth } from "@/lib/api-handler";
+import { annotate } from "@/lib/logging/context";
+import { apiSuccess, returnAllZodIssues } from "@/lib/api-response";
+import { requireIllnessEnabled } from "@/lib/illness/gate";
+import { illnessInsightsQuerySchema } from "@/lib/validations/illness";
+import { computeEpisodeCorrelation } from "@/lib/illness/correlation-read";
+import {
+  summarizeIllnessRetrospective,
+  type RetrospectiveEpisode,
+} from "@/lib/illness/retrospective";
+import { dayKeyForUserTz } from "@/lib/measurements/consolidation-tz";
+import { resolveUserTimezone } from "@/lib/measurements/consolidation-base";
+
+const MS_PER_DAY = 24 * 60 * 60 * 1000;
+/** Cap the per-episode gap computations so a heavy history can't fan out. */
+const MAX_GAP_COMPUTATIONS = 24;
+
+export const GET = apiHandler(async (request: NextRequest) => {
+  const { user } = await requireAuth();
+
+  const gate = await requireIllnessEnabled(user.id);
+  if (!gate.enabled) return gate.response;
+
+  const parsed = illnessInsightsQuerySchema.safeParse({
+    windowDays: new URL(request.url).searchParams.get("windowDays") ?? undefined,
+  });
+  if (!parsed.success) {
+    return returnAllZodIssues(parsed.error, 422, {
+      errorCode: "illness.insights.invalid",
+    });
+  }
+
+  const windowDays = parsed.data.windowDays ?? 365;
+  const now = new Date();
+  const since = new Date(now.getTime() - windowDays * MS_PER_DAY);
+  const tz = resolveUserTimezone(user.timezone ?? null);
+
+  const rows = await prisma.illnessEpisode.findMany({
+    where: { userId: user.id, deletedAt: null, onsetAt: { gte: since } },
+    orderBy: { onsetAt: "desc" },
+    select: {
+      id: true,
+      type: true,
+      lifecycle: true,
+      onsetAt: true,
+      resolvedAt: true,
+    },
+  });
+
+  // Compute the recovery-gap only for resolved, non-chronic episodes (the
+  // only ones that CAN have a gap), bounded to MAX_GAP_COMPUTATIONS most
+  // recent. Each gap comes from the coverage-gated engine — a thin episode
+  // yields null and simply doesn't contribute to the typical-gap median.
+  const gapCandidates = rows
+    .filter((r) => r.resolvedAt !== null && r.lifecycle !== "CHRONIC_ONGOING")
+    .slice(0, MAX_GAP_COMPUTATIONS);
+  const gapById = new Map<string, number | null>();
+  for (const r of gapCandidates) {
+    const derived = await computeEpisodeCorrelation(
+      user.id,
+      { id: r.id, onsetAt: r.onsetAt, resolvedAt: r.resolvedAt, lifecycle: r.lifecycle },
+      tz,
+      now,
+    );
+    gapById.set(
+      r.id,
+      derived.status === "ok" ? derived.value.recoveryGapDays : null,
+    );
+  }
+
+  const episodes: RetrospectiveEpisode[] = rows.map((r) => ({
+    id: r.id,
+    type: r.type,
+    onsetDay: dayKeyForUserTz(r.onsetAt, tz),
+    resolved: r.resolvedAt !== null,
+    recoveryGapDays: gapById.get(r.id) ?? null,
+    lifecycle: r.lifecycle,
+  }));
+
+  const summary = summarizeIllnessRetrospective(episodes);
+
+  annotate({
+    action: { name: "illness.insights.read", entity_type: "illness_episode" },
+    meta: {
+      episode_count: summary.episodeCount,
+      gap_sample_size: summary.gapSampleSize,
+      typical_gap_days: summary.typicalRecoveryGapDays,
+    },
+  });
+
+  return apiSuccess({ windowDays, ...summary });
+});
diff --git a/src/lib/illness/__tests__/correlation-read.test.ts b/src/lib/illness/__tests__/correlation-read.test.ts
new file mode 100644
index 000000000..01c1625a5
--- /dev/null
+++ b/src/lib/illness/__tests__/correlation-read.test.ts
@@ -0,0 +1,210 @@
+/**
+ * v1.18.1 P3 — illness correlation READ layer + rollup-vs-live parity.
+ *
+ * The reliability anchor: the same per-day series fed to the engine must
+ * produce IDENTICAL findings whether it was resolved from the rollup DAY tier
+ * or the live-SQL fallback. We drive `computeEpisodeCorrelation` twice over the
+ * SAME underlying daily data — once with full rollup coverage (the DAY path),
+ * once with a coverage miss (the live raw-read path) — and assert the engine
+ * output is byte-identical (modulo the provenance `source` tag).
+ *
+ * Also pins the contamination-guard READ window: the baseline read window must
+ * end strictly before the pre-onset lookback (no episode-span day reaches the
+ * baseline query).
+ */
+import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
+
+/* The DAY-native daily means the test universe holds for RESTING_HEART_RATE.
+ * Baseline window sits at ~55; the episode-span (from 01-03 onward) spikes.
+ * Onset 2026-01-10, lookback 7 → pre-onset starts 2026-01-03; baseline window
+ * is the 30 days ending 2026-01-03. */
+const DAILY: Array<{ day: string; mean: number }> = [];
+{
+  // 30 clean baseline days ending the day before 2026-01-03.
+  let cursor = Date.parse("2026-01-02T00:00:00Z");
+  // Even spread around 55 so the MAD (and band) is non-zero and stable.
+  const offsets = [-2, -1, 0, 1, 2];
+  for (let i = 0; i < 30; i++) {
+    DAILY.unshift({
+      day: new Date(cursor).toISOString().slice(0, 10),
+      mean: 55 + offsets[i % offsets.length],
+    });
+    cursor -= 24 * 60 * 60 * 1000;
+  }
+  // Episode span.
+  for (const [day, mean] of [
+    ["2026-01-03", 56],
+    ["2026-01-08", 70], // pre-onset notable
+    ["2026-01-10", 74],
+    ["2026-01-12", 76], // nadir
+    ["2026-01-15", 64],
+    ["2026-01-17", 58],
+    ["2026-01-18", 55],
+    ["2026-01-19", 55],
+    ["2026-01-20", 55],
+  ] as const) {
+    DAILY.push({ day, mean });
+  }
+}
+
+const NOW = new Date("2026-01-21T12:00:00Z");
+
+const db = vi.hoisted(() => ({
+  measurement: { findMany: vi.fn() },
+}));
+const rollup = vi.hoisted(() => ({
+  probeRollupCoverage: vi.fn(),
+  readBestGranularityRollups: vi.fn(),
+}));
+
+vi.mock("@/lib/db", () => ({ prisma: db }));
+vi.mock("@/lib/rollups/measurement-coverage", () => ({
+  probeRollupCoverage: rollup.probeRollupCoverage,
+}));
+vi.mock("@/lib/rollups/measurement-read-wmy", () => ({
+  readBestGranularityRollups: rollup.readBestGranularityRollups,
+}));
+
+import { computeEpisodeCorrelation } from "../correlation-read";
+import { ILLNESS_SCAN_TYPES } from "../correlation";
+
+/**
+ * An ACTIVE episode (no resolvedAt) so the episode-window read's `to` is
+ * effectively `now` — this exercises the trailing rollup DAY path in the
+ * covered case and the trailing live path in the miss case, the exact seam
+ * the parity test must pin. The baseline window stays bounded (raw both ways).
+ */
+const EPISODE = {
+  id: "ep-parity",
+  onsetAt: new Date("2026-01-10T08:00:00Z"),
+  resolvedAt: null as Date | null,
+  lifecycle: "ACUTE",
+};
+
+/** Daily rows within [from, to) as a raw-measurement findMany result. */
+function rawRowsWithin(from: Date, to: Date) {
+  const fromKey = from.toISOString().slice(0, 10);
+  const toKey = to.toISOString().slice(0, 10);
+  return DAILY.filter((d) => d.day >= fromKey && d.day < toKey).map((d) => ({
+    value: d.mean,
+    measuredAt: new Date(`${d.day}T12:00:00Z`),
+  }));
+}
+
+beforeEach(() => {
+  vi.clearAllMocks();
+});
+afterEach(() => {
+  vi.restoreAllMocks();
+});
+
+describe("computeEpisodeCorrelation — rollup-vs-live parity", () => {
+  function onlyRhr<T>(map: Map<string, T>): Map<string, T> {
+    return map;
+  }
+
+  it("produces identical findings on the DAY path and the live path", async () => {
+    // ── DAY path: full coverage for RESTING_HEART_RATE, rollup serves the
+    //    trailing (to == now) episode-window read; the bounded baseline read
+    //    goes through measurement.findMany (to != now). ──
+    const dayCoverage = onlyRhr(
+      new Map<string, boolean>(ILLNESS_SCAN_TYPES.map((t) => [String(t), t === "RESTING_HEART_RATE"])),
+    );
+    rollup.probeRollupCoverage.mockResolvedValue(dayCoverage);
+    rollup.readBestGranularityRollups.mockImplementation(
+      async (_u: string, type: string, windowDays: number) => {
+        if (type !== "RESTING_HEART_RATE") return null;
+        const since = new Date(NOW.getTime() - windowDays * 24 * 60 * 60 * 1000);
+        const sinceKey = since.toISOString().slice(0, 10);
+        const rows = DAILY.filter((d) => d.day >= sinceKey).map((d) => ({
+          bucketStart: new Date(`${d.day}T00:00:00Z`),
+          count: 1,
+          mean: d.mean,
+          sd: null,
+          slope: null,
+          r2: null,
+          sumValue: null,
+          minValue: d.mean,
+          maxValue: d.mean,
+        }));
+        return rows.length > 0 ? { granularity: "DAY" as const, rows } : null;
+      },
+    );
+    db.measurement.findMany.mockImplementation(async ({ where }: { where: { type: string; measuredAt: { gte: Date; lt: Date } } }) => {
+      if (where.type !== "RESTING_HEART_RATE") return [];
+      return rawRowsWithin(where.measuredAt.gte, where.measuredAt.lt);
+    });
+
+    const dayOut = await computeEpisodeCorrelation("u1", EPISODE, "UTC", NOW);
+
+    // ── Live path: zero rollup coverage, every read falls to findMany. The
+    //    baseline engine's live fallback groups raw rows itself for the
+    //    trailing read; we serve the same DAILY universe for any window. ──
+    vi.clearAllMocks();
+    const noCoverage = new Map<string, boolean>(
+      ILLNESS_SCAN_TYPES.map((t) => [String(t), false]),
+    );
+    rollup.probeRollupCoverage.mockResolvedValue(noCoverage);
+    rollup.readBestGranularityRollups.mockResolvedValue(null);
+    db.measurement.findMany.mockImplementation(
+      async ({ where }: { where: { type: string; measuredAt: { gte: Date; lt?: Date } } }) => {
+        if (where.type !== "RESTING_HEART_RATE") return [];
+        const from = where.measuredAt.gte;
+        const to = where.measuredAt.lt ?? NOW;
+        return rawRowsWithin(from, to);
+      },
+    );
+
+    const liveOut = await computeEpisodeCorrelation("u1", EPISODE, "UTC", NOW);
+
+    expect(dayOut.status).toBe("ok");
+    expect(liveOut.status).toBe("ok");
+    if (dayOut.status !== "ok" || liveOut.status !== "ok") return;
+
+    // Same numbers regardless of read path (the parity guarantee): the
+    // DAY-rollup-fed series and the live-SQL series produce byte-identical
+    // findings.
+    expect(liveOut.value.recoveryGapDays).toBe(dayOut.value.recoveryGapDays);
+    expect(liveOut.value.nadir).toEqual(dayOut.value.nadir);
+    expect(liveOut.value.preOnset).toEqual(dayOut.value.preOnset);
+    expect(liveOut.value.returns).toEqual(dayOut.value.returns);
+
+    // And the findings are the expected golden values.
+    expect(dayOut.value.nadir[0]?.day).toBe("2026-01-12");
+    expect(dayOut.value.nadir[0]?.value).toBe(76);
+    expect(dayOut.value.preOnset[0]?.day).toBe("2026-01-08");
+    // Active episode → no felt-better marker → the gap is null but the
+    // physiological return is still detected (01-17 is the first in-band day
+    // that then holds for ≥3 observed days).
+    expect(dayOut.value.recoveryGapDays).toBeNull();
+    expect(dayOut.value.returns[0]?.returnedDay).toBe("2026-01-17");
+  });
+});
+
+describe("computeEpisodeCorrelation — contamination-guard read window", () => {
+  it("never queries baseline rows on or after the pre-onset lookback start", async () => {
+    rollup.probeRollupCoverage.mockResolvedValue(
+      new Map<string, boolean>(ILLNESS_SCAN_TYPES.map((t) => [String(t), false])),
+    );
+    rollup.readBestGranularityRollups.mockResolvedValue(null);
+    const seen: Array<{ gte: Date; lt?: Date }> = [];
+    db.measurement.findMany.mockImplementation(
+      async ({ where }: { where: { type: string; measuredAt: { gte: Date; lt?: Date } } }) => {
+        if (where.type === "RESTING_HEART_RATE") seen.push(where.measuredAt);
+        return [];
+      },
+    );
+
+    await computeEpisodeCorrelation("u1", EPISODE, "UTC", NOW);
+
+    // The pre-onset lookback starts at onset − 7 days = 2026-01-03.
+    const lookbackStart = Date.parse("2026-01-03T08:00:00Z");
+    // The bounded baseline read (the one with a `lt`) must END at/ before
+    // the lookback start — no baseline query may reach into the episode span.
+    const boundedBaseline = seen.filter((w) => w.lt !== undefined);
+    expect(boundedBaseline.length).toBeGreaterThan(0);
+    for (const w of boundedBaseline) {
+      expect(w.lt!.getTime()).toBeLessThanOrEqual(lookbackStart);
+    }
+  });
+});
diff --git a/src/lib/illness/__tests__/correlation.test.ts b/src/lib/illness/__tests__/correlation.test.ts
new file mode 100644
index 000000000..5c627d887
--- /dev/null
+++ b/src/lib/illness/__tests__/correlation.test.ts
@@ -0,0 +1,294 @@
+/**
+ * v1.18.1 P3 — illness correlation engine (the pure math).
+ *
+ * The reliability battery for `computeIllnessCorrelation`:
+ *   - golden-fixture recovery-gap math (felt-better vs physiological return)
+ *   - baseline-contamination guard (episode days must not poison the baseline)
+ *   - thin-data / coverage-gate (asserts NOTHING below the floor)
+ *   - pre-onset anomaly scan + nadir direction (adverse vs not)
+ *   - red-flag escalation (sustained SpO2 drop / fever)
+ *   - CHRONIC_ONGOING exclusion from the gap
+ *
+ * Pure — no DB. Series are hand-built so each finding is exactly predictable.
+ */
+import { describe, expect, it } from "vitest";
+import type { MeasurementType } from "@/generated/prisma/client";
+import {
+  computeIllnessCorrelation,
+  dayDiff,
+  MIN_BASELINE_DAYS,
+  type IllnessCorrelationInput,
+  type VitalDayPoint,
+} from "../correlation";
+
+const NOW = new Date("2026-02-01T00:00:00Z");
+
+/** Build a flat baseline series of `days` days at `value`, ending `endDay`. */
+function flatBaseline(value: number, days: number, endDay: string): VitalDayPoint[] {
+  const out: VitalDayPoint[] = [];
+  let cursor = Date.parse(`${endDay}T00:00:00Z`);
+  for (let i = 0; i < days; i++) {
+    out.unshift({ day: new Date(cursor).toISOString().slice(0, 10), mean: value });
+    cursor -= 24 * 60 * 60 * 1000;
+  }
+  return out;
+}
+
+/**
+ * A baseline whose per-day means spread evenly so the MAD (and thus the band)
+ * is non-zero and stable. Means cycle through `center − jitter … center +
+ * jitter` so the median is `center` and the MAD is ~`jitter/2`.
+ */
+function jitteredBaseline(center: number, jitter: number, days: number, endDay: string): VitalDayPoint[] {
+  const base = flatBaseline(center, days, endDay);
+  const offsets = [-jitter, -jitter / 2, 0, jitter / 2, jitter];
+  return base.map((p, i) => ({ ...p, mean: center + offsets[i % offsets.length] }));
+}
+
+function input(over: Partial<IllnessCorrelationInput>): IllnessCorrelationInput {
+  return {
+    episodeId: "ep1",
+    window: { onsetDay: "2026-01-10", feltBetterDay: "2026-01-17", lifecycle: "ACUTE" },
+    series: [],
+    source: "DAY",
+    now: NOW,
+    ...over,
+  };
+}
+
+describe("computeIllnessCorrelation — coverage gate", () => {
+  it("returns insufficient with no baselined vitals", () => {
+    const out = computeIllnessCorrelation(input({ series: [] }));
+    expect(out.status).toBe("insufficient");
+    if (out.status === "insufficient") expect(out.reason).toBe("no_baselined_vitals");
+  });
+
+  it("returns insufficient when a vital lacks the min baseline days", () => {
+    const type: MeasurementType = "RESTING_HEART_RATE";
+    const out = computeIllnessCorrelation(
+      input({
+        series: [
+          {
+            type,
+            baselineDays: flatBaseline(55, MIN_BASELINE_DAYS - 1, "2026-01-02"),
+            episodeDays: flatBaseline(55, 6, "2026-01-15"),
+          },
+        ],
+      }),
+    );
+    expect(out.status).toBe("insufficient");
+  });
+
+  it("returns insufficient when episode coverage is below the floor", () => {
+    const type: MeasurementType = "RESTING_HEART_RATE";
+    const out = computeIllnessCorrelation(
+      input({
+        series: [
+          {
+            type,
+            baselineDays: jitteredBaseline(55, 1, 14, "2026-01-02"),
+            episodeDays: flatBaseline(55, 2, "2026-01-11"), // only 2 episode days
+          },
+        ],
+      }),
+    );
+    expect(out.status).toBe("insufficient");
+    if (out.status === "insufficient")
+      expect(out.reason).toBe("insufficient_episode_coverage");
+  });
+});
+
+describe("computeIllnessCorrelation — golden recovery-gap", () => {
+  it("computes a positive gap when the body lags the felt-better marker", () => {
+    // RHR baseline 55, MAD-band ≈ ±4.4 (spread ≈ 2.22, NOTABLE = 2σ). Onset
+    // 01-10, felt better 01-17. RHR spikes to 75, stays clearly out-of-band
+    // (≥62) through 01-19, then re-enters the band and holds from 01-20.
+    const type: MeasurementType = "RESTING_HEART_RATE";
+    const baselineDays = jitteredBaseline(55, 1, 21, "2026-01-02");
+    const episodeDays: VitalDayPoint[] = [
+      { day: "2026-01-09", mean: 55 }, // pre-onset, in band
+      { day: "2026-01-10", mean: 70 },
+      { day: "2026-01-12", mean: 75 }, // nadir
+      { day: "2026-01-15", mean: 68 },
+      { day: "2026-01-17", mean: 64 }, // felt-better day, still out-of-band
+      { day: "2026-01-19", mean: 62 }, // still out-of-band
+      { day: "2026-01-20", mean: 55 }, // return start (in band)
+      { day: "2026-01-21", mean: 55 },
+      { day: "2026-01-22", mean: 55 }, // stable 3 days
+    ];
+    const out = computeIllnessCorrelation(
+      input({ series: [{ type, baselineDays, episodeDays }] }),
+    );
+    expect(out.status).toBe("ok");
+    if (out.status !== "ok") return;
+    // returned 01-20, felt better 01-17 → gap +3
+    expect(out.value.recoveryGapDays).toBe(dayDiff("2026-01-17", "2026-01-20"));
+    expect(out.value.recoveryGapDays).toBe(3);
+    // nadir at 01-12, value 75, adverse (RHR up = adverse)
+    expect(out.value.nadir[0]?.day).toBe("2026-01-12");
+    expect(out.value.nadir[0]?.value).toBe(75);
+    expect(out.value.nadir[0]?.adverse).toBe(true);
+    expect(out.value.nadir[0]?.direction).toBe("above");
+  });
+
+  it("flags a pre-onset anomaly when the body deviated before onset", () => {
+    const type: MeasurementType = "HEART_RATE_VARIABILITY";
+    // HRV baseline 60±2. Drops to 30 two days before onset → adverse (down).
+    const baselineDays = jitteredBaseline(60, 2, 21, "2026-01-02");
+    const episodeDays: VitalDayPoint[] = [
+      { day: "2026-01-08", mean: 30 }, // pre-onset crash (notable)
+      { day: "2026-01-10", mean: 28 },
+      { day: "2026-01-12", mean: 32 },
+      { day: "2026-01-14", mean: 45 },
+      { day: "2026-01-15", mean: 60 },
+    ];
+    const out = computeIllnessCorrelation(
+      input({ series: [{ type, baselineDays, episodeDays }] }),
+    );
+    expect(out.status).toBe("ok");
+    if (out.status !== "ok") return;
+    expect(out.value.preOnset[0]?.day).toBe("2026-01-08");
+    expect(out.value.preOnset[0]?.direction).toBe("below");
+    expect(out.value.preOnset[0]?.adverse).toBe(true);
+  });
+
+  it("excludes CHRONIC_ONGOING from the recovery-gap", () => {
+    const type: MeasurementType = "RESTING_HEART_RATE";
+    const baselineDays = jitteredBaseline(55, 1, 21, "2026-01-02");
+    const episodeDays: VitalDayPoint[] = [
+      { day: "2026-01-10", mean: 70 },
+      { day: "2026-01-12", mean: 75 },
+      { day: "2026-01-15", mean: 55 },
+      { day: "2026-01-16", mean: 55 },
+      { day: "2026-01-17", mean: 55 },
+    ];
+    const out = computeIllnessCorrelation(
+      input({
+        window: { onsetDay: "2026-01-10", feltBetterDay: "2026-01-15", lifecycle: "CHRONIC_ONGOING" },
+        series: [{ type, baselineDays, episodeDays }],
+      }),
+    );
+    expect(out.status).toBe("ok");
+    if (out.status !== "ok") return;
+    expect(out.value.recoveryGapDays).toBeNull();
+    expect(out.value.returns).toHaveLength(0);
+  });
+
+  it("leaves the gap null when the vital never stably returns (still active)", () => {
+    const type: MeasurementType = "RESTING_HEART_RATE";
+    const baselineDays = jitteredBaseline(55, 1, 21, "2026-01-02");
+    const episodeDays: VitalDayPoint[] = [
+      { day: "2026-01-10", mean: 72 },
+      { day: "2026-01-12", mean: 75 },
+      { day: "2026-01-14", mean: 73 },
+      { day: "2026-01-16", mean: 70 }, // never returns to band
+    ];
+    const out = computeIllnessCorrelation(
+      input({
+        window: { onsetDay: "2026-01-10", feltBetterDay: null, lifecycle: "ACUTE" },
+        series: [{ type, baselineDays, episodeDays }],
+      }),
+    );
+    expect(out.status).toBe("ok");
+    if (out.status !== "ok") return;
+    expect(out.value.recoveryGapDays).toBeNull();
+    expect(out.value.returns[0]?.returnedDay).toBeNull();
+  });
+});
+
+describe("computeIllnessCorrelation — baseline-contamination guard", () => {
+  it("uses ONLY baselineDays for the band, never the episode-span values", () => {
+    // If episode-span spikes contaminated the band, the 75 readings would
+    // widen the band and the deviation would shrink. We assert the band is
+    // built purely from baselineDays by comparing two episodes whose
+    // baselineDays are identical but episodeDays differ wildly — the nadir
+    // deviationSd for the SAME nadir value must be identical.
+    const type: MeasurementType = "RESTING_HEART_RATE";
+    const baselineDays = jitteredBaseline(55, 1, 21, "2026-01-02");
+
+    const calm = computeIllnessCorrelation(
+      input({
+        series: [
+          {
+            type,
+            baselineDays,
+            episodeDays: [
+              { day: "2026-01-10", mean: 56 },
+              { day: "2026-01-11", mean: 75 }, // single nadir
+              { day: "2026-01-12", mean: 56 },
+              { day: "2026-01-13", mean: 55 },
+            ],
+          },
+        ],
+      }),
+    );
+    const stormy = computeIllnessCorrelation(
+      input({
+        series: [
+          {
+            type,
+            baselineDays,
+            episodeDays: [
+              { day: "2026-01-10", mean: 90 },
+              { day: "2026-01-11", mean: 75 }, // SAME nadir value as calm
+              { day: "2026-01-12", mean: 88 },
+              { day: "2026-01-13", mean: 92 },
+            ],
+          },
+        ],
+      }),
+    );
+    expect(calm.status).toBe("ok");
+    expect(stormy.status).toBe("ok");
+    if (calm.status !== "ok" || stormy.status !== "ok") return;
+    // The band center is the clean baseline median (55) in BOTH episodes —
+    // the wildly different episode-span values never shift it. A contaminated
+    // baseline (one that ingested the 75–92 readings) would pull the center
+    // upward and the two centers would diverge.
+    const calmCenter = calm.value.nadir[0]?.baselineCenter;
+    const stormyCenter = stormy.value.nadir[0]?.baselineCenter;
+    expect(calmCenter).toBe(55);
+    expect(stormyCenter).toBe(55);
+    expect(stormyCenter).toBe(calmCenter);
+  });
+});
+
+describe("computeIllnessCorrelation — red-flag escalation", () => {
+  it("escalates a sustained low SpO2 run", () => {
+    const type: MeasurementType = "OXYGEN_SATURATION";
+    const baselineDays = jitteredBaseline(98, 1, 21, "2026-01-02");
+    const episodeDays: VitalDayPoint[] = [
+      { day: "2026-01-10", mean: 91 },
+      { day: "2026-01-11", mean: 90 },
+      { day: "2026-01-12", mean: 89 }, // 3 consecutive ≤92
+      { day: "2026-01-13", mean: 96 },
+      { day: "2026-01-14", mean: 98 },
+    ];
+    const out = computeIllnessCorrelation(
+      input({ series: [{ type, baselineDays, episodeDays }] }),
+    );
+    expect(out.status).toBe("ok");
+    if (out.status !== "ok") return;
+    expect(out.value.redFlags).toHaveLength(1);
+    expect(out.value.redFlags[0].reason).toBe("sustained_low_spo2");
+    expect(out.value.redFlags[0].worstValue).toBe(89);
+    expect(out.value.redFlags[0].days).toBe(3);
+  });
+
+  it("does not escalate a single low SpO2 day", () => {
+    const type: MeasurementType = "OXYGEN_SATURATION";
+    const baselineDays = jitteredBaseline(98, 1, 21, "2026-01-02");
+    const episodeDays: VitalDayPoint[] = [
+      { day: "2026-01-10", mean: 91 }, // one dip only
+      { day: "2026-01-11", mean: 97 },
+      { day: "2026-01-12", mean: 98 },
+      { day: "2026-01-13", mean: 98 },
+    ];
+    const out = computeIllnessCorrelation(
+      input({ series: [{ type, baselineDays, episodeDays }] }),
+    );
+    expect(out.status).toBe("ok");
+    if (out.status !== "ok") return;
+    expect(out.value.redFlags).toHaveLength(0);
+  });
+});
diff --git a/src/lib/illness/__tests__/retrospective.test.ts b/src/lib/illness/__tests__/retrospective.test.ts
new file mode 100644
index 000000000..1c143b71d
--- /dev/null
+++ b/src/lib/illness/__tests__/retrospective.test.ts
@@ -0,0 +1,73 @@
+/**
+ * v1.18.1 P3 — illness retrospective summary aggregation.
+ *
+ * Asserts the count/byMonth/byType tallies, the CHRONIC_ONGOING gap exclusion,
+ * and the thin-data gate: the "typical recovery gap" is WITHHELD below the
+ * min-sample floor (never a "typical" claim from one episode).
+ */
+import { describe, expect, it } from "vitest";
+import {
+  summarizeIllnessRetrospective,
+  MIN_EPISODES_FOR_TYPICAL_GAP,
+  type RetrospectiveEpisode,
+} from "../retrospective";
+
+function ep(over: Partial<RetrospectiveEpisode>): RetrospectiveEpisode {
+  return {
+    id: "e",
+    type: "INFECTION",
+    onsetDay: "2026-01-10",
+    resolved: true,
+    recoveryGapDays: 2,
+    lifecycle: "ACUTE",
+    ...over,
+  };
+}
+
+describe("summarizeIllnessRetrospective", () => {
+  it("counts episodes, resolved, by-month, by-type", () => {
+    const out = summarizeIllnessRetrospective([
+      ep({ id: "a", onsetDay: "2026-01-05", type: "INFECTION" }),
+      ep({ id: "b", onsetDay: "2026-11-20", type: "INFECTION", resolved: false, recoveryGapDays: null }),
+      ep({ id: "c", onsetDay: "2026-11-02", type: "ALLERGY" }),
+    ]);
+    expect(out.episodeCount).toBe(3);
+    expect(out.resolvedCount).toBe(2);
+    expect(out.byMonth[1]).toBe(1);
+    expect(out.byMonth[11]).toBe(2);
+    expect(out.byType.INFECTION).toBe(2);
+    expect(out.byType.ALLERGY).toBe(1);
+  });
+
+  it("withholds the typical gap below the min-sample floor", () => {
+    const out = summarizeIllnessRetrospective([
+      ep({ id: "a", recoveryGapDays: 3 }),
+      ep({ id: "b", recoveryGapDays: 5 }),
+    ]);
+    expect(MIN_EPISODES_FOR_TYPICAL_GAP).toBe(3);
+    expect(out.gapSampleSize).toBe(2);
+    expect(out.typicalRecoveryGapDays).toBeNull();
+  });
+
+  it("reports the median typical gap at/above the floor", () => {
+    const out = summarizeIllnessRetrospective([
+      ep({ id: "a", recoveryGapDays: 2 }),
+      ep({ id: "b", recoveryGapDays: 4 }),
+      ep({ id: "c", recoveryGapDays: 6 }),
+    ]);
+    expect(out.gapSampleSize).toBe(3);
+    expect(out.typicalRecoveryGapDays).toBe(4);
+  });
+
+  it("excludes CHRONIC_ONGOING and null gaps from the median", () => {
+    const out = summarizeIllnessRetrospective([
+      ep({ id: "a", recoveryGapDays: 2 }),
+      ep({ id: "b", recoveryGapDays: 4 }),
+      ep({ id: "c", recoveryGapDays: 6 }),
+      ep({ id: "d", recoveryGapDays: 100, lifecycle: "CHRONIC_ONGOING" }),
+      ep({ id: "e", recoveryGapDays: null }),
+    ]);
+    expect(out.gapSampleSize).toBe(3);
+    expect(out.typicalRecoveryGapDays).toBe(4);
+  });
+});
diff --git a/src/lib/illness/correlation-read.ts b/src/lib/illness/correlation-read.ts
new file mode 100644
index 000000000..29667cefb
--- /dev/null
+++ b/src/lib/illness/correlation-read.ts
@@ -0,0 +1,168 @@
+/**
+ * Illness correlation engine — the READ layer (v1.18.1, Workstream B / P3).
+ *
+ * Resolves the per-vital per-day mean series the pure `computeIllnessCorrelation`
+ * engine needs, then runs the engine. Two windows per vital, and the seam
+ * between them is the BASELINE-CONTAMINATION GUARD:
+ *
+ *   baselineDays : [onset − lookback − BASELINE_WINDOW_DAYS, onset − lookback)
+ *   episodeDays  : [onset − lookback, end]
+ *
+ * The baseline window ENDS strictly before the pre-onset lookback, so no day
+ * inside (or just before) the episode can poison the baseline the episode is
+ * measured against. This is the single most important reliability property of
+ * the engine — a baseline that includes the illness days would shrink every
+ * deviation toward zero and silently under-report the very anomaly we exist to
+ * surface.
+ *
+ * Reads are DAY-native (the spread invariant forbids composing a band from a
+ * coarser tier's `sd`): the rollup DAY tier when covered, a bounded per-type
+ * live-SQL fallback on a coverage miss. The `readDayMeanSeries` helper the
+ * baseline engine already exports does exactly this and is reused verbatim, so
+ * the rollup-vs-live parity the baseline tests pin carries over for free.
+ */
+import type { MeasurementType } from "@/generated/prisma/client";
+import { prisma } from "@/lib/db";
+import { readDayMeanSeries } from "@/lib/insights/derived/baseline";
+import { probeRollupCoverage } from "@/lib/rollups/measurement-coverage";
+import { dayKeyForUserTz } from "@/lib/measurements/consolidation-tz";
+import { resolveUserTimezone } from "@/lib/measurements/consolidation-base";
+import type { Derived } from "@/lib/insights/derived/types";
+import {
+  BASELINE_WINDOW_DAYS,
+  ILLNESS_SCAN_TYPES,
+  PRE_ONSET_LOOKBACK_DAYS,
+  computeIllnessCorrelation,
+  type IllnessCorrelationValue,
+  type VitalSeries,
+} from "./correlation";
+
+const MS_PER_DAY = 24 * 60 * 60 * 1000;
+
+/** The episode shape the read layer needs (a subset of the Prisma row). */
+export interface EpisodeForCorrelation {
+  id: string;
+  onsetAt: Date;
+  resolvedAt: Date | null;
+  lifecycle: string;
+}
+
+/**
+ * Resolve the per-vital series for one episode (contamination-guarded
+ * baseline window) and run the pure engine. Returns the same gated
+ * `Derived<T>` the engine returns — `insufficient` flows straight through.
+ */
+export async function computeEpisodeCorrelation(
+  userId: string,
+  episode: EpisodeForCorrelation,
+  timezone: string | null,
+  now: Date = new Date(),
+): Promise<Derived<IllnessCorrelationValue>> {
+  const tz = resolveUserTimezone(timezone);
+  const onset = episode.onsetAt;
+  const end = episode.resolvedAt ?? now;
+
+  // Window seams (UTC instants; the day keys are read in the user's tz).
+  const preOnsetStart = new Date(onset.getTime() - PRE_ONSET_LOOKBACK_DAYS * MS_PER_DAY);
+  const baselineEnd = preOnsetStart; // baseline ends where the lookback starts
+  const baselineStart = new Date(
+    baselineEnd.getTime() - BASELINE_WINDOW_DAYS * MS_PER_DAY,
+  );
+
+  const onsetDay = dayKeyForUserTz(onset, tz);
+  const feltBetterDay = episode.resolvedAt
+    ? dayKeyForUserTz(episode.resolvedAt, tz)
+    : null;
+
+  // One coverage probe shared across every vital read (pool-contention
+  // mitigation — the same pattern the derived route uses).
+  const coverage = await probeRollupCoverage(userId);
+
+  const series: VitalSeries[] = [];
+  let anyLive = false;
+  let anyDay = false;
+
+  // We attempt every scan type. The read helper reuses the baseline engine's
+  // rollup-first path (live fallback on a coverage miss); an untracked vital
+  // returns empty points and is dropped below. Each read is a single windowed
+  // query, so attempting all types is bounded.
+  for (const type of ILLNESS_SCAN_TYPES) {
+    const baseline = await readWindowMeans(userId, type, baselineStart, baselineEnd, coverage, now);
+    const episodeWin = await readWindowMeans(userId, type, preOnsetStart, end, coverage, now);
+    if (baseline.points.length === 0 && episodeWin.points.length === 0) continue;
+    if (baseline.source === "live" || episodeWin.source === "live") anyLive = true;
+    if (baseline.source === "DAY" || episodeWin.source === "DAY") anyDay = true;
+    series.push({
+      type,
+      baselineDays: baseline.points.map((p) => ({ day: p.day, mean: p.mean })),
+      episodeDays: episodeWin.points.map((p) => ({ day: p.day, mean: p.mean })),
+    });
+  }
+
+  const source: "DAY" | "live" | "none" = anyDay
+    ? "DAY"
+    : anyLive
+      ? "live"
+      : "none";
+
+  return computeIllnessCorrelation({
+    episodeId: episode.id,
+    window: { onsetDay, feltBetterDay, lifecycle: episode.lifecycle },
+    series,
+    source,
+    now,
+  });
+}
+
+/**
+ * Per-day means for `(userId, type)` over an explicit [from, to) window.
+ * Reuses the baseline engine's `readDayMeanSeries` (DAY-native rollup with a
+ * live fallback) for the whole-window read, then trims to the window so the
+ * baseline and episode windows can share the one cached read path while
+ * keeping their seams exact. The trim is on the local day key so the
+ * contamination guard holds in the user's timezone.
+ */
+async function readWindowMeans(
+  userId: string,
+  type: MeasurementType,
+  from: Date,
+  to: Date,
+  coverage: Awaited<ReturnType<typeof probeRollupCoverage>>,
+  now: Date,
+): Promise<{ points: { day: string; mean: number }[]; source: "DAY" | "live" | "none" }> {
+  // The baseline helper reads a trailing window ending at `now`. We need an
+  // arbitrary [from, to) window, so read directly here: DAY rollup first via
+  // the helper for the trailing case, else a bounded raw read. To keep the
+  // seam exact we always do the bounded raw per-day grouping for the bounded
+  // window (a single indexed query), and only use the helper's rollup path
+  // when `to` is effectively `now` (the common active-episode case).
+  const toIsNow = Math.abs(to.getTime() - now.getTime()) < MS_PER_DAY;
+  if (toIsNow) {
+    const windowDays = Math.max(1, Math.ceil((to.getTime() - from.getTime()) / MS_PER_DAY));
+    const resolved = await readDayMeanSeries(userId, type, windowDays, now, coverage);
+    const fromKey = from.toISOString().slice(0, 10);
+    return {
+      points: resolved.points.filter((p) => p.day >= fromKey),
+      source: resolved.source === "DAY" ? "DAY" : resolved.source === "none" ? "none" : "live",
+    };
+  }
+
+  const rows = await prisma.measurement.findMany({
+    where: { userId, type, deletedAt: null, measuredAt: { gte: from, lt: to } },
+    orderBy: { measuredAt: "asc" },
+    select: { value: true, measuredAt: true },
+  });
+  if (rows.length === 0) return { points: [], source: "none" };
+  const byDay = new Map<string, { sum: number; count: number }>();
+  for (const row of rows) {
+    const day = row.measuredAt.toISOString().slice(0, 10);
+    const acc = byDay.get(day) ?? { sum: 0, count: 0 };
+    acc.sum += row.value;
+    acc.count += 1;
+    byDay.set(day, acc);
+  }
+  const points = [...byDay.entries()]
+    .sort(([a], [b]) => (a < b ? -1 : a > b ? 1 : 0))
+    .map(([day, acc]) => ({ day, mean: acc.sum / acc.count }));
+  return { points, source: "live" };
+}
diff --git a/src/lib/illness/correlation.ts b/src/lib/illness/correlation.ts
new file mode 100644
index 000000000..af7ad174d
--- /dev/null
+++ b/src/lib/illness/correlation.ts
@@ -0,0 +1,508 @@
+/**
+ * Illness correlation + recovery-gap engine (v1.18.1, Workstream B / P3).
+ *
+ * Server-authoritative, reliability-first. Given ONE illness episode this
+ * assembles three retrospective findings — never a prediction, never a
+ * diagnosis — each gated by the shared coverage/confidence model so a thin
+ * signal asserts NOTHING:
+ *
+ *   1. **Pre-onset anomaly scan** — the N days BEFORE onset, each tracked
+ *      vital's deviation in robust SDs from the user's OWN baseline (median
+ *      ± MAD, the same `computeVitalsBaseline` engine), the baseline computed
+ *      from a window that ENDS before the pre-onset lookback so the episode's
+ *      own physiology can never poison the baseline it is measured against.
+ *      "How did it announce itself?"
+ *   2. **Nadir / what-dropped** — across the episode's active span, the
+ *      furthest each vital strayed from baseline (the nadir day + magnitude +
+ *      direction). "What dropped."
+ *   3. **Recovery-gap** — the intellectual core: physiological return-to-
+ *      baseline (first day a previously-deviated vital re-enters its band and
+ *      STAYS in for `RETURN_STABILITY_DAYS`) vs the felt-better marker
+ *      (`resolvedAt`). The gap is `physiologicalReturn − feltBetter` in days:
+ *      positive = the body lagged the feeling (you felt better before your
+ *      numbers did), negative = numbers normalised first. CHRONIC_ONGOING is
+ *      excluded (no recovered date).
+ *
+ * The directionality of "worse" is metric-specific (a HRV/recovery DROP and a
+ * resting-HR/temperature RISE both mean "stressed"); `ADVERSE_DIRECTION`
+ * pins it per metric. Red-flag patterns (a sustained large oxygen-saturation
+ * drop, a sustained fever) ESCALATE to a "seek care" flag — never reassure.
+ *
+ * Pure given its inputs: the caller does the bounded reads (baseline series +
+ * per-day vital means within the episode window) and passes them in, so the
+ * whole engine is unit-testable against golden fixtures with no DB. The route
+ * wires the reads; the parity test asserts the rollup-fed path and a raw-SQL
+ * path agree.
+ */
+import type { MeasurementType } from "@/generated/prisma/client";
+import { buildBaselineBand, median } from "@/lib/insights/derived/baseline";
+import {
+  buildInsufficient,
+  buildOk,
+  deriveCoverage,
+  nowProvenanceTimestamp,
+} from "@/lib/insights/derived/coverage";
+import type { Derived } from "@/lib/insights/derived/types";
+
+/* ── tunables (documented invariants) ────────────────────────────────── */
+
+/** Days before onset scanned for the "how did it announce itself" signal. */
+export const PRE_ONSET_LOOKBACK_DAYS = 7;
+/** Trailing baseline window (days), ending BEFORE the pre-onset lookback. */
+export const BASELINE_WINDOW_DAYS = 30;
+/**
+ * Minimum distinct baseline days a vital needs before we trust its band.
+ * Mirrors `computeVitalsBaseline`'s 7-day floor — below this the vital is
+ * dropped from the scan rather than asserted on a fragile band.
+ */
+export const MIN_BASELINE_DAYS = 7;
+/**
+ * A deviation of ≥ this many robust SDs (k·MAD·scale units) from baseline
+ * counts as "notable" for the pre-onset + nadir surfaces. ~2σ-equivalent.
+ */
+export const NOTABLE_SD = 2;
+/**
+ * Consecutive in-band days required to call a vital "returned to baseline"
+ * for the recovery-gap (one lucky in-band day is not a recovery).
+ */
+export const RETURN_STABILITY_DAYS = 3;
+/** Minimum episode days with ANY vital coverage before the gap is asserted. */
+export const MIN_EPISODE_COVERAGE_DAYS = 4;
+
+/**
+ * Per-metric adverse direction — which way a reading moves when the body is
+ * under illness stress. `up` = a RISE is adverse (resting HR, temperature,
+ * respiratory rate, BP); `down` = a DROP is adverse (HRV, recovery, SpO2).
+ * Weight/glucose have no single illness-adverse direction → omitted (their
+ * deviation magnitude is still surfaced, direction stays neutral).
+ */
+export const ADVERSE_DIRECTION: Partial<Record<MeasurementType, "up" | "down">> =
+  {
+    RESTING_HEART_RATE: "up",
+    PULSE: "up",
+    BODY_TEMPERATURE: "up",
+    SKIN_TEMPERATURE: "up",
+    RESPIRATORY_RATE: "up",
+    BLOOD_PRESSURE_SYS: "up",
+    BLOOD_PRESSURE_DIA: "up",
+    HEART_RATE_VARIABILITY: "down",
+    RECOVERY_SCORE: "down",
+    OXYGEN_SATURATION: "down",
+  };
+
+/**
+ * Vitals scanned by the illness engine. Superset of the typical-range set
+ * plus RECOVERY_SCORE (a canonical wellness signal). The caller only passes
+ * series for the vitals the user actually tracks; the rest are skipped.
+ */
+export const ILLNESS_SCAN_TYPES: MeasurementType[] = [
+  "RESTING_HEART_RATE",
+  "HEART_RATE_VARIABILITY",
+  "RECOVERY_SCORE",
+  "OXYGEN_SATURATION",
+  "BODY_TEMPERATURE",
+  "RESPIRATORY_RATE",
+  "BLOOD_PRESSURE_SYS",
+  "BLOOD_PRESSURE_DIA",
+  "PULSE",
+  "WEIGHT",
+];
+
+/* ── inputs the caller resolves and passes in (pure engine) ──────────── */
+
+/** A per-day mean for one vital (YYYY-MM-DD local day → mean). */
+export interface VitalDayPoint {
+  /** Local day key, `YYYY-MM-DD`. */
+  day: string;
+  /** Mean reading for the day. */
+  mean: number;
+}
+
+/**
+ * Everything the engine needs for ONE vital: the baseline-window series
+ * (ENDS before the pre-onset lookback — contamination guard lives in the
+ * caller's read window) and the episode-window series (pre-onset lookback
+ * through the felt-better marker, inclusive).
+ */
+export interface VitalSeries {
+  type: MeasurementType;
+  /** Per-day means over the clean baseline window (no episode-span days). */
+  baselineDays: VitalDayPoint[];
+  /** Per-day means from `onset − lookback` through `resolvedAt` (or today). */
+  episodeDays: VitalDayPoint[];
+}
+
+/** The dates the engine reasons over, all `YYYY-MM-DD` local day keys. */
+export interface EpisodeWindow {
+  /** Onset day. */
+  onsetDay: string;
+  /** Felt-better marker (resolvedAt local day), or null when still active. */
+  feltBetterDay: string | null;
+  /** Lifecycle — CHRONIC_ONGOING suppresses the recovery-gap. */
+  lifecycle: string;
+}
+
+export interface IllnessCorrelationInput {
+  episodeId: string;
+  window: EpisodeWindow;
+  /** One entry per vital the user tracks (others omitted by the caller). */
+  series: VitalSeries[];
+  /** Provenance source the dominant read resolved against. */
+  source: "DAY" | "live" | "none";
+  /** Compute time (injected for deterministic tests). */
+  now?: Date;
+}
+
+/* ── output value (the server-authoritative DTO payload) ─────────────── */
+
+/** One vital's deviation finding on a given day. */
+export interface VitalDeviationFinding {
+  type: MeasurementType;
+  /** The day this finding describes (`YYYY-MM-DD`). */
+  day: string;
+  /** The day's mean reading. */
+  value: number;
+  /** Robust baseline center the deviation is measured from. */
+  baselineCenter: number;
+  /** Signed deviation in robust-SD (k·MAD·scale) units; +above / −below. */
+  deviationSd: number;
+  /** Which way the reading moved. */
+  direction: "above" | "below";
+  /** True when this move is the illness-adverse direction for the metric. */
+  adverse: boolean;
+}
+
+/** A vital's recovery timing within the episode. */
+export interface VitalReturnFinding {
+  type: MeasurementType;
+  /** First day the vital re-entered its band AND stayed in for the window. */
+  returnedDay: string | null;
+  /** Days from felt-better to physiological return (signed); null if N/A. */
+  gapDays: number | null;
+}
+
+export interface IllnessCorrelationValue {
+  episodeId: string;
+  /** "How did it announce itself" — notable pre-onset deviations. */
+  preOnset: VitalDeviationFinding[];
+  /** "What dropped" — each vital's worst (nadir) deviation in the episode. */
+  nadir: VitalDeviationFinding[];
+  /** Per-vital physiological-return timing. */
+  returns: VitalReturnFinding[];
+  /**
+   * The headline recovery-gap in days (median of the per-vital gaps), or
+   * null when CHRONIC_ONGOING / still active / no vital returned. Positive
+   * = the body lagged the felt-better marker.
+   */
+  recoveryGapDays: number | null;
+  /** Felt-better marker echoed for the surface. */
+  feltBetterDay: string | null;
+  /**
+   * Red-flag escalation — a sustained adverse SpO2 drop or sustained fever
+   * during the episode. Retrospective, but the copy must escalate ("if this
+   * recurs, seek care"), NEVER reassure. Empty when nothing tripped.
+   */
+  redFlags: IllnessRedFlag[];
+}
+
+export interface IllnessRedFlag {
+  type: MeasurementType;
+  /** A short stable reason key the i18n layer renders (not free text). */
+  reason: "sustained_low_spo2" | "sustained_fever";
+  /** The worst observed value during the flagged run. */
+  worstValue: number;
+  /** Consecutive days the adverse threshold held. */
+  days: number;
+}
+
+/* ── red-flag thresholds (clinical floors, conservative) ─────────────── */
+
+/** SpO2 at/below this for ≥ RED_FLAG_RUN_DAYS escalates. */
+const SPO2_RED_FLAG = 92;
+/** Body temperature at/above this (°C) for ≥ RED_FLAG_RUN_DAYS escalates. */
+const FEVER_RED_FLAG = 38.5;
+/** Consecutive days an adverse clinical threshold must hold to escalate. */
+const RED_FLAG_RUN_DAYS = 3;
+
+/* ── pure helpers ─────────────────────────────────────────────────────── */
+
+/** Robust band for a vital, or null below the min-baseline-days floor. */
+function bandFor(series: VitalSeries): {
+  center: number;
+  spread: number;
+} | null {
+  if (series.baselineDays.length < MIN_BASELINE_DAYS) return null;
+  const band = buildBaselineBand(series.baselineDays.map((p) => p.mean));
+  if (!band || band.spread <= 0) return null;
+  return { center: band.center, spread: band.spread };
+}
+
+/** Signed deviation of a value in robust-SD units (spread = k·MAD·scale). */
+function deviationSd(value: number, center: number, spread: number): number {
+  return (value - center) / spread;
+}
+
+/** Build a deviation finding for one day/value against a band. */
+function finding(
+  type: MeasurementType,
+  day: string,
+  value: number,
+  center: number,
+  spread: number,
+): VitalDeviationFinding {
+  const sd = deviationSd(value, center, spread);
+  const direction: "above" | "below" = sd >= 0 ? "above" : "below";
+  const adverseDir = ADVERSE_DIRECTION[type];
+  const adverse =
+    (adverseDir === "up" && direction === "above") ||
+    (adverseDir === "down" && direction === "below");
+  return {
+    type,
+    day,
+    value,
+    baselineCenter: center,
+    deviationSd: Math.round(sd * 100) / 100,
+    direction,
+    adverse,
+  };
+}
+
+/** Order day keys chronologically. */
+function byDay(a: { day: string }, b: { day: string }): number {
+  return a.day < b.day ? -1 : a.day > b.day ? 1 : 0;
+}
+
+/* ── the engine ───────────────────────────────────────────────────────── */
+
+/**
+ * Compute the retrospective correlation findings for one episode. Pure given
+ * its inputs — the caller resolves the reads (rollup tier with a live-SQL
+ * fallback) and passes the series in. Returns a gated `Derived<T>`: below the
+ * coverage floor it returns `insufficient` (never a fabricated finding).
+ */
+export function computeIllnessCorrelation(
+  input: IllnessCorrelationInput,
+): Derived<IllnessCorrelationValue> {
+  const now = input.now ?? new Date();
+  const computedAt = nowProvenanceTimestamp(now);
+  const { window } = input;
+
+  // Only vitals with a trustworthy own-baseline band participate.
+  const banded = input.series
+    .map((s) => ({ series: s, band: bandFor(s) }))
+    .filter((b): b is { series: VitalSeries; band: { center: number; spread: number } } => b.band !== null);
+
+  const inputs = banded.map((b) => String(b.series.type));
+  const provenance = {
+    inputs,
+    source: input.source,
+    windowDays: BASELINE_WINDOW_DAYS,
+    computedAt,
+  };
+
+  // Coverage: how many distinct episode days carry ANY banded vital reading.
+  const episodeDayKeys = new Set<string>();
+  for (const b of banded) {
+    for (const p of b.series.episodeDays) {
+      if (p.day >= window.onsetDay) episodeDayKeys.add(p.day);
+    }
+  }
+  const coverageDays = episodeDayKeys.size;
+
+  if (banded.length === 0 || coverageDays < MIN_EPISODE_COVERAGE_DAYS) {
+    const { coverage } = deriveCoverage({
+      requiredInputs: 1,
+      presentInputs: banded.length,
+      historyDays: coverageDays,
+      missing: banded.length === 0 ? ILLNESS_SCAN_TYPES.map(String) : [],
+      fullHistoryDays: MIN_EPISODE_COVERAGE_DAYS,
+    });
+    return buildInsufficient<IllnessCorrelationValue>({
+      coverage,
+      provenance,
+      reason:
+        banded.length === 0
+          ? "no_baselined_vitals"
+          : "insufficient_episode_coverage",
+    });
+  }
+
+  const preOnset: VitalDeviationFinding[] = [];
+  const nadir: VitalDeviationFinding[] = [];
+  const returns: VitalReturnFinding[] = [];
+  const redFlags: IllnessRedFlag[] = [];
+
+  for (const { series, band } of banded) {
+    const { type } = series;
+    const { center, spread } = band;
+    const episode = [...series.episodeDays].sort(byDay);
+
+    // 1) Pre-onset scan: lookback window strictly before onset.
+    const pre = episode.filter((p) => p.day < window.onsetDay);
+    let worstPre: VitalDeviationFinding | null = null;
+    for (const p of pre) {
+      const f = finding(type, p.day, p.mean, center, spread);
+      if (
+        Math.abs(f.deviationSd) >= NOTABLE_SD &&
+        (worstPre === null ||
+          Math.abs(f.deviationSd) > Math.abs(worstPre.deviationSd))
+      ) {
+        worstPre = f;
+      }
+    }
+    if (worstPre) preOnset.push(worstPre);
+
+    // 2) Nadir: worst deviation across the active span (onset → end).
+    const active = episode.filter((p) => p.day >= window.onsetDay);
+    let worstActive: VitalDeviationFinding | null = null;
+    for (const p of active) {
+      const f = finding(type, p.day, p.mean, center, spread);
+      if (
+        Math.abs(f.deviationSd) >= NOTABLE_SD &&
+        (worstActive === null ||
+          Math.abs(f.deviationSd) > Math.abs(worstActive.deviationSd))
+      ) {
+        worstActive = f;
+      }
+    }
+    if (worstActive) nadir.push(worstActive);
+
+    // 3) Physiological return: first active day this vital deviated
+    //    notably-adverse, then later re-entered the band and STAYED in for
+    //    RETURN_STABILITY_DAYS. Only meaningful if it actually deviated.
+    const deviatedActive = active.some((p) => {
+      const f = finding(type, p.day, p.mean, center, spread);
+      return Math.abs(f.deviationSd) >= NOTABLE_SD;
+    });
+    if (deviatedActive && window.lifecycle !== "CHRONIC_ONGOING") {
+      const returnedDay = firstStableReturn(active, center, spread);
+      const gapDays =
+        returnedDay && window.feltBetterDay
+          ? dayDiff(window.feltBetterDay, returnedDay)
+          : null;
+      returns.push({ type, returnedDay, gapDays });
+    }
+
+    // Red-flag escalation (clinical floors, retrospective).
+    const flag = detectRedFlag(type, active);
+    if (flag) redFlags.push(flag);
+  }
+
+  // Headline gap = median of the per-vital signed gaps (robust to one vital
+  // recovering oddly). Null for CHRONIC_ONGOING / still active / no returns.
+  const gaps = returns
+    .map((r) => r.gapDays)
+    .filter((g): g is number => g !== null);
+  const recoveryGapDays =
+    window.lifecycle === "CHRONIC_ONGOING" || gaps.length === 0
+      ? null
+      : Math.round(median(gaps));
+
+  const { coverage, confidence } = deriveCoverage({
+    requiredInputs: 1,
+    presentInputs: banded.length,
+    historyDays: coverageDays,
+    missing: [],
+    fullHistoryDays: Math.max(MIN_EPISODE_COVERAGE_DAYS, 14),
+  });
+
+  return buildOk<IllnessCorrelationValue>({
+    value: {
+      episodeId: input.episodeId,
+      preOnset: preOnset.sort((a, b) => Math.abs(b.deviationSd) - Math.abs(a.deviationSd)),
+      nadir: nadir.sort((a, b) => Math.abs(b.deviationSd) - Math.abs(a.deviationSd)),
+      returns,
+      recoveryGapDays,
+      feltBetterDay: window.feltBetterDay,
+      redFlags,
+    },
+    coverage,
+    confidence,
+    provenance,
+  });
+}
+
+/**
+ * First day in `active` (chronological) where the vital is in-band AND stays
+ * in-band for the next `RETURN_STABILITY_DAYS` observed days. Null when it
+ * never settles. "In-band" = |deviation| < NOTABLE_SD.
+ */
+function firstStableReturn(
+  active: VitalDayPoint[],
+  center: number,
+  spread: number,
+): string | null {
+  const inBand = active.map((p) => ({
+    day: p.day,
+    in: Math.abs(deviationSd(p.mean, center, spread)) < NOTABLE_SD,
+  }));
+  for (let i = 0; i < inBand.length; i++) {
+    if (!inBand[i].in) continue;
+    let run = 0;
+    for (let j = i; j < inBand.length && inBand[j].in; j++) run++;
+    if (run >= RETURN_STABILITY_DAYS) return inBand[i].day;
+  }
+  return null;
+}
+
+/**
+ * Detect a retrospective red flag: a sustained adverse SpO2 drop or a
+ * sustained fever during the active span. Conservative clinical floors;
+ * escalation copy lives in the i18n layer.
+ */
+function detectRedFlag(
+  type: MeasurementType,
+  active: VitalDayPoint[],
+): IllnessRedFlag | null {
+  const sorted = [...active].sort(byDay);
+  if (type === "OXYGEN_SATURATION") {
+    return runFlag(sorted, (v) => v <= SPO2_RED_FLAG, "sustained_low_spo2", type, "min");
+  }
+  if (type === "BODY_TEMPERATURE") {
+    return runFlag(sorted, (v) => v >= FEVER_RED_FLAG, "sustained_fever", type, "max");
+  }
+  return null;
+}
+
+/** Longest consecutive run matching `predicate`; flag when ≥ RED_FLAG_RUN_DAYS. */
+function runFlag(
+  sorted: VitalDayPoint[],
+  predicate: (v: number) => boolean,
+  reason: IllnessRedFlag["reason"],
+  type: MeasurementType,
+  worst: "min" | "max",
+): IllnessRedFlag | null {
+  let bestRun = 0;
+  let bestWorst: number | null = null;
+  let run = 0;
+  let runWorst: number | null = null;
+  for (const p of sorted) {
+    if (predicate(p.mean)) {
+      run++;
+      runWorst =
+        runWorst === null
+          ? p.mean
+          : worst === "min"
+            ? Math.min(runWorst, p.mean)
+            : Math.max(runWorst, p.mean);
+      if (run > bestRun) {
+        bestRun = run;
+        bestWorst = runWorst;
+      }
+    } else {
+      run = 0;
+      runWorst = null;
+    }
+  }
+  if (bestRun >= RED_FLAG_RUN_DAYS && bestWorst !== null) {
+    return { type, reason, worstValue: bestWorst, days: bestRun };
+  }
+  return null;
+}
+
+/** Whole-day signed difference `to − from` for two `YYYY-MM-DD` keys. */
+export function dayDiff(from: string, to: string): number {
+  const a = Date.parse(`${from}T00:00:00Z`);
+  const b = Date.parse(`${to}T00:00:00Z`);
+  return Math.round((b - a) / (24 * 60 * 60 * 1000));
+}
diff --git a/src/lib/illness/retrospective.ts b/src/lib/illness/retrospective.ts
new file mode 100644
index 000000000..08ab725f3
--- /dev/null
+++ b/src/lib/illness/retrospective.ts
@@ -0,0 +1,98 @@
+/**
+ * Illness retrospective summary (v1.18.1, Workstream B / P3).
+ *
+ * The look-back surface: "sick N times this year · typical recovery gap X
+ * days". RETROSPECTIVE ONLY — it summarises what already happened. It is
+ * NEVER a predictor or diagnoser; the recurrence figure is a count of past
+ * episodes, not a forecast, and the copy layer must frame it as such.
+ *
+ * Pure given a list of resolved episode summaries + their computed gaps. The
+ * caller resolves the per-episode recovery-gap (via the correlation engine,
+ * which is itself coverage-gated) and passes the numbers in; this layer only
+ * aggregates. The aggregate is itself gated — with too few resolved episodes
+ * to say anything honest, it reports the count but withholds the "typical
+ * gap" (asserts nothing thin).
+ */
+import { median } from "@/lib/insights/derived/baseline";
+
+/** Minimum resolved-with-gap episodes before a "typical gap" is asserted. */
+export const MIN_EPISODES_FOR_TYPICAL_GAP = 3;
+
+/** One episode's contribution to the summary. */
+export interface RetrospectiveEpisode {
+  id: string;
+  type: string;
+  /** Onset local day `YYYY-MM-DD` (for the recurrence-month tally). */
+  onsetDay: string;
+  /** Whether the episode is resolved (has a felt-better marker). */
+  resolved: boolean;
+  /** The computed recovery-gap in days, or null when ungated/unavailable. */
+  recoveryGapDays: number | null;
+  /** Lifecycle — CHRONIC_ONGOING never contributes a gap. */
+  lifecycle: string;
+}
+
+export interface IllnessRetrospectiveSummary {
+  /** Episodes whose onset falls in the requested window (e.g. trailing year). */
+  episodeCount: number;
+  /** Of those, how many are resolved. */
+  resolvedCount: number;
+  /**
+   * Typical (median) recovery-gap in days across episodes with a computed
+   * gap — null when fewer than `MIN_EPISODES_FOR_TYPICAL_GAP` qualify. A
+   * positive gap means the body typically lagged the felt-better marker.
+   */
+  typicalRecoveryGapDays: number | null;
+  /** How many episodes backed the typical-gap figure (transparency). */
+  gapSampleSize: number;
+  /**
+   * Recurrence tally by calendar month (1–12) → count, retrospective only.
+   * Drives "you tend to log illnesses in November" as an OBSERVATION, never
+   * a prediction. Empty when nothing falls in the window.
+   */
+  byMonth: Record<number, number>;
+  /** Per-type episode counts in the window (retrospective breakdown). */
+  byType: Record<string, number>;
+}
+
+/**
+ * Aggregate the per-episode figures into the retrospective summary. Pure. The
+ * typical-gap is withheld (null) below the min-sample floor so a single
+ * episode never becomes a "typical" claim.
+ */
+export function summarizeIllnessRetrospective(
+  episodes: RetrospectiveEpisode[],
+): IllnessRetrospectiveSummary {
+  const byMonth: Record<number, number> = {};
+  const byType: Record<string, number> = {};
+  let resolvedCount = 0;
+  const gaps: number[] = [];
+
+  for (const e of episodes) {
+    const month = Number(e.onsetDay.slice(5, 7));
+    if (month >= 1 && month <= 12) byMonth[month] = (byMonth[month] ?? 0) + 1;
+    byType[e.type] = (byType[e.type] ?? 0) + 1;
+    if (e.resolved) resolvedCount++;
+    if (
+      e.lifecycle !== "CHRONIC_ONGOING" &&
+      e.recoveryGapDays !== null &&
+      Number.isFinite(e.recoveryGapDays)
+    ) {
+      gaps.push(e.recoveryGapDays);
+    }
+  }
+
+  const typicalRecoveryGapDays =
+    gaps.length >= MIN_EPISODES_FOR_TYPICAL_GAP
+      ? Math.round(median(gaps))
+      : null;
+
+  return {
+    episodeCount: episodes.length,
+    resolvedCount,
+    typicalRecoveryGapDays,
+    gapSampleSize: gaps.length,
+    byMonth,
+    byType,
+  };
+}
diff --git a/src/lib/query-keys/illness.ts b/src/lib/query-keys/illness.ts
index 9c2276915..1b1d9ee80 100644
--- a/src/lib/query-keys/illness.ts
+++ b/src/lib/query-keys/illness.ts
@@ -18,4 +18,17 @@ export const illnessKeys = {
   illnessEpisode: (id: string) => ["illness", "episode", id] as const,
   illnessDayLog: (episodeId: string, date: string) =>
     ["illness", "day-log", episodeId, date] as const,
+  /**
+   * v1.18.1 P3 — the per-episode retrospective correlation findings
+   * (pre-onset scan, nadir, recovery-gap). Server-authoritative; the read
+   * is gated, so the surface pattern-matches `status` rather than recomputing.
+   */
+  illnessCorrelation: (id: string) =>
+    ["illness", "correlation", id] as const,
+  /**
+   * v1.18.1 P3 — the cross-episode retrospective summary ("sick N times ·
+   * typical recovery gap X days") over a trailing window in days.
+   */
+  illnessInsights: (windowDays: number) =>
+    ["illness", "insights", windowDays] as const,
 };
diff --git a/src/lib/validations/illness.ts b/src/lib/validations/illness.ts
index b97de9018..5af3fc615 100644
--- a/src/lib/validations/illness.ts
+++ b/src/lib/validations/illness.ts
@@ -133,3 +133,14 @@ export type IllnessDayLogInput = z.infer<typeof illnessDayLogInputSchema>;
 
 /** The single-day read query: `GET .../day-logs?date=YYYY-MM-DD`. */
 export const illnessDayLogQuerySchema = z.object({ date: dateString });
+
+/* ── P3 retrospective insight window ─────────────────────────────────── */
+
+/**
+ * The retrospective-insight window query: `GET /api/illness/insights?
+ * windowDays=365`. Bounded 30..1095 days; defaults to a trailing year.
+ * Retrospective only — the engine summarises past episodes, never forecasts.
+ */
+export const illnessInsightsQuerySchema = z.object({
+  windowDays: z.coerce.number().int().min(30).max(1095).optional(),
+});