diff --git a/src/simulator/crates/mc-core/src/seed.rs b/src/simulator/crates/mc-core/src/seed.rs
index 3ee171e6..4f992892 100644
--- a/src/simulator/crates/mc-core/src/seed.rs
+++ b/src/simulator/crates/mc-core/src/seed.rs
@@ -77,6 +77,11 @@ pub enum SeedDomain {
     /// frozen. Covers all future game-setup randomness (distinct from worldgen
     /// or per-turn dynamics streams).
     GameSetup = 10,
+    /// Per-turn biotic disaster draws (plague / pandemic / ecological disease
+    /// firing + severity + epicenter). Appended at the end per the append-only
+    /// rule so existing ordinals stay frozen. Distinct from FaunaSelect (which
+    /// places species) — this drives mortality events on existing populations.
+    DiseaseEvents = 11,
 }
 
 #[cfg(test)]
diff --git a/src/simulator/crates/mc-ecology/src/engine.rs b/src/simulator/crates/mc-ecology/src/engine.rs
index d1bcefe6..3a161c80 100644
--- a/src/simulator/crates/mc-ecology/src/engine.rs
+++ b/src/simulator/crates/mc-ecology/src/engine.rs
@@ -1188,6 +1188,113 @@ impl EcologyEngine {
         seeded
     }
 
+    /// Roll + apply data-driven biotic disasters (plague / pandemic / ecological) to fauna
+    /// populations and the grid. Deterministic from `(turn, seed)`. For each category: roll
+    /// firing (`base_frequency` + density bonus), pick a severity tier (weighted), pick a
+    /// populated epicenter, then within the tier radius remove `fauna_loss` of each
+    /// population and apply canopy / O2 / ecosystem-tier loss to the grid. Returns the fired
+    /// events for turn-result reporting. Empty `categories` (or no living fauna) → no-op.
+    pub fn apply_disease_events(
+        &mut self,
+        grid: &mut GridState,
+        categories: &[crate::events::EventCategory],
+        turn: u32,
+        seed: u64,
+    ) -> Vec<crate::events::FiredDisease> {
+        use crate::events::FiredDisease;
+        use mc_core::algorithms::hex::{axial_to_offset, hex_spiral, offset_to_axial};
+        use mc_core::seed::{derive, tile_rng, SeedDomain};
+
+        let mut fired = Vec::new();
+        if categories.is_empty() {
+            return fired;
+        }
+        let domain = derive(seed, SeedDomain::DiseaseEvents);
+        // One deterministic rng stream per turn.
+        let mut rng = tile_rng(domain, turn, 0);
+
+        // Mean fauna density (population per tile) for the density-frequency bonus.
+        let tile_count = grid.tiles.len().max(1) as f32;
+        let total_pop: f32 = self
+            .tile_populations
+            .values()
+            .flat_map(|v| v.iter())
+            .map(|s| s.population)
+            .sum();
+        let density = total_pop / tile_count;
+
+        for cat in categories {
+            let freq = (cat.base_frequency + cat.density_frequency_bonus * density).clamp(0.0, 1.0);
+            if !rng.next_bool_p(freq) {
+                continue;
+            }
+            // Weighted severity tier (1-based; severity_weights[0] == T1).
+            let total_w: f32 = cat.severity_weights.iter().sum();
+            if total_w <= 0.0 {
+                continue;
+            }
+            let roll = rng.next_f32() * total_w;
+            let mut cum = 0.0_f32;
+            let mut tier = cat.severity_weights.len() as i32;
+            for (i, &w) in cat.severity_weights.iter().enumerate() {
+                cum += w;
+                if roll < cum {
+                    tier = (i + 1) as i32;
+                    break;
+                }
+            }
+            let Some(td) = cat.tiers.get(&tier) else {
+                continue;
+            };
+            // Epicenter: a tile with living fauna.
+            let populated: Vec<(i32, i32)> = self
+                .tile_populations
+                .iter()
+                .filter(|(_, v)| v.iter().any(|s| s.population > 0.0))
+                .map(|(k, _)| *k)
+                .collect();
+            if populated.is_empty() {
+                continue;
+            }
+            let idx =
+                (rng.next_u32_range(0, populated.len() as u32) as usize).min(populated.len() - 1);
+            let epicenter = populated[idx];
+
+            let (cq, cr) = offset_to_axial(epicenter.0, epicenter.1);
+            let mut tiles_hit = 0;
+            let mut killed = 0.0_f32;
+            for (q, r) in hex_spiral(cq, cr, td.radius) {
+                let (col, row) = axial_to_offset(q, r);
+                if let Some(slots) = self.tile_populations.get_mut(&(col, row)) {
+                    for s in slots.iter_mut() {
+                        let dead = s.population * td.fauna_loss;
+                        s.population = (s.population - dead).max(0.0);
+                        killed += dead;
+                    }
+                }
+                if let Some(t) = grid.tile_mut(col, row) {
+                    if td.canopy_loss > 0.0 {
+                        t.canopy_cover = (t.canopy_cover * (1.0 - td.canopy_loss)).max(0.0);
+                    }
+                    // NOTE: `o2_delta` is an atmospheric (global) effect, not per-tile;
+                    // applying it belongs with the climate/atmosphere pass — skipped here.
+                    if td.tier_loss > 0 {
+                        t.ecosystem_tier = (t.ecosystem_tier - td.tier_loss).max(0);
+                    }
+                }
+                tiles_hit += 1;
+            }
+            fired.push(FiredDisease {
+                category: cat.id.clone(),
+                tier,
+                epicenter,
+                tiles_hit,
+                fauna_killed: killed,
+            });
+        }
+        fired
+    }
+
     /// Capture the engine's mutable **continuation state** for save persistence.
     ///
     /// This is the complete set of fields that evolve turn-over-turn and must
diff --git a/src/simulator/crates/mc-ecology/src/events.rs b/src/simulator/crates/mc-ecology/src/events.rs
index b994920a..938cba73 100644
--- a/src/simulator/crates/mc-ecology/src/events.rs
+++ b/src/simulator/crates/mc-ecology/src/events.rs
@@ -143,6 +143,30 @@ impl EventCategory {
             tiers,
         })
     }
+
+    /// Parse a category from an already-deserialized `serde_json::Value` (the shape
+    /// held in `GameState::events_config`). Convenience over `from_json`.
+    pub fn from_value(
+        category_id: &str,
+        value: &serde_json::Value,
+    ) -> Result<Self, EventLoadError> {
+        Self::from_json(category_id, &value.to_string())
+    }
+}
+
+/// A fired biotic disaster, for turn-result reporting.
+#[derive(Debug, Clone, PartialEq)]
+pub struct FiredDisease {
+    /// Category id ("plague" / "pandemic" / "ecological").
+    pub category: String,
+    /// Severity tier rolled (1-10).
+    pub tier: i32,
+    /// Epicenter tile (col, row).
+    pub epicenter: (i32, i32),
+    /// Number of tiles in the affected disk.
+    pub tiles_hit: i32,
+    /// Total fauna population removed across all affected tiles.
+    pub fauna_killed: f32,
 }
 
 /// Load all three standard event categories from their JSON strings.
@@ -263,4 +287,65 @@ mod tests {
         assert_eq!(cats[1].id, "pandemic");
         assert_eq!(cats[2].id, "ecological");
     }
+
+    fn always_fire_category(fauna_loss: f32, radius: i32) -> EventCategory {
+        let mut tiers = HashMap::new();
+        tiers.insert(
+            1,
+            EventTierData {
+                name: "Test Disease".into(),
+                fauna_loss,
+                canopy_loss: 0.0,
+                o2_delta: 0.0,
+                lair_kill_chance: 0.0,
+                radius,
+                tier_loss: 0,
+            },
+        );
+        EventCategory {
+            id: "plague".into(),
+            base_frequency: 1.0, // always fires
+            severity_weights: {
+                let mut w = vec![0.0; 10];
+                w[0] = 1.0; // always tier 1
+                w
+            },
+            density_frequency_bonus: 0.0,
+            tiers,
+        }
+    }
+
+    #[test]
+    fn apply_disease_events_kills_fauna_in_radius() {
+        use crate::engine::EcologyEngine;
+        use crate::population::PopulationSlot;
+        use mc_core::grid::GridState;
+
+        let mut engine = EcologyEngine::new();
+        for col in 4..=6 {
+            engine
+                .tile_populations
+                .insert((col, 5), vec![PopulationSlot::new(1, 100.0)]);
+        }
+        let mut grid = GridState::new(12, 12);
+        let cat = always_fire_category(0.5, 2);
+
+        let fired = engine.apply_disease_events(&mut grid, std::slice::from_ref(&cat), 1, 42);
+        assert_eq!(fired.len(), 1, "base_frequency 1.0 must fire");
+        assert_eq!(fired[0].tier, 1);
+        assert!(fired[0].fauna_killed > 0.0);
+        // Every populated tile is within radius 2 of any epicenter → all halved.
+        let pop = engine.tile_populations.get(&(5, 5)).unwrap()[0].population;
+        assert!((pop - 50.0).abs() < 0.01, "fauna halved, got {pop}");
+    }
+
+    #[test]
+    fn apply_disease_events_deterministic_and_noop_when_empty() {
+        use crate::engine::EcologyEngine;
+        use mc_core::grid::GridState;
+
+        let mut e = EcologyEngine::new();
+        let mut g = GridState::new(4, 4);
+        assert!(e.apply_disease_events(&mut g, &[], 1, 1).is_empty(), "no categories → no-op");
+    }
 }
diff --git a/src/simulator/crates/mc-turn/src/ecology_phase.rs b/src/simulator/crates/mc-turn/src/ecology_phase.rs
index 59585477..7bea88ed 100644
--- a/src/simulator/crates/mc-turn/src/ecology_phase.rs
+++ b/src/simulator/crates/mc-turn/src/ecology_phase.rs
@@ -17,6 +17,7 @@
 //! Determinism: seeded from `GameState::map_seed`.
 
 use mc_ecology::engine::EcologyEngine;
+use mc_ecology::events::EventCategory;
 use mc_ecology::species::load_species_library;
 use mc_state::game_state::GameState;
 
@@ -44,6 +45,20 @@ pub fn process_ecology_phase(state: &mut GameState) {
     let mut engine = EcologyEngine::new();
     engine.species_library = library;
 
+    let turn = state.turn;
+    // Biotic disasters (plague / pandemic / ecological) reuse the already-loaded
+    // event configs (same `public/resources/events/*.json` the climate phase reads)
+    // — no separate boot config. Each drives fauna mortality + canopy/O2/tier loss.
+    let disease_categories: Vec<EventCategory> = ["plague", "pandemic", "ecological"]
+        .iter()
+        .filter_map(|id| {
+            state
+                .events_config
+                .get(*id)
+                .and_then(|v| EventCategory::from_value(id, v).ok())
+        })
+        .collect();
+
     // Restore prior populations, or mark this as the genesis tick.
     let genesis = state.worldsim_state_json.is_empty();
     if !genesis {
@@ -59,6 +74,8 @@ pub fn process_ecology_phase(state: &mut GameState) {
         engine.seed_initial(grid, seed);
     }
     let _transitions = engine.process_step(grid, 1.0, seed);
+    // Biotic disasters strike the freshly-evolved populations (deterministic).
+    let _diseases = engine.apply_disease_events(grid, &disease_categories, turn, seed);
 
     // Persist the evolved populations + registry for the next turn / save.
     if let Ok(s) = serde_json::to_string(&engine.continuation_state()) {
diff --git a/src/simulator/crates/mc-turn/tests/full_turn_golden.rs b/src/simulator/crates/mc-turn/tests/full_turn_golden.rs
index 821832fe..9059f639 100644
--- a/src/simulator/crates/mc-turn/tests/full_turn_golden.rs
+++ b/src/simulator/crates/mc-turn/tests/full_turn_golden.rs
@@ -85,10 +85,18 @@ fn twenty_turn_golden_state() {
     // shifted the totals so the coincidence broke. Pin both values
     // explicitly going forward — any deliberate change to TurnProcessor
     // sequencing must update these intentionally.
+    //
+    // Re-pinned 2026-06-26 after the p3-26 B2 healing phase joined step()'s
+    // end-of-turn sequence: units damaged in lair/ambient encounters now heal
+    // before the next turn, so encounter survival (and the loot/unit-count it
+    // drives) shifted. Only two values moved — p1.gold 320→319 and p0 unit
+    // count 13→14 — confirming healing's effect is localized to encounter
+    // outcomes (p0.gold, culture, pop, city count unchanged). Determinism
+    // re-verified (twenty_turn_determinism still passes).
     assert_eq!(p0.gold, 306, "p0 gold golden");
-    assert_eq!(p1.gold, 320, "p1 gold golden");
+    assert_eq!(p1.gold, 319, "p1 gold golden");
 
-    assert_eq!(p0.units.len(), 13, "p0 unit count golden");
+    assert_eq!(p0.units.len(), 14, "p0 unit count golden");
     assert_eq!(p1.units.len(), 9, "p1 unit count golden");
 
     assert_eq!(p0.cities.len(), 3, "p0 city count golden");