diff --git a/src/status.lua b/src/status.lua
new file mode 100644
index 0000000..e57c671
--- /dev/null
+++ b/src/status.lua
@@ -0,0 +1,114 @@
+-- Data-driven status effects. Each effect ticks once when its host takes a turn:
+-- damage-over-time drains hp, regeneration restores it, stun skips the action,
+-- and haste/slow/strength feed modifiers into speed and combat. Effects are
+-- plain data on entity.statuses so they serialize without special handling.
+
+local Status = {}
+
+Status.POISON = "poison"
+Status.BURNING = "burning"
+Status.STUN = "stun"
+Status.REGEN = "regen"
+Status.HASTE = "haste"
+Status.SLOW = "slow"
+Status.STRENGTH = "strength"
+
+Status.defs = {
+  [Status.POISON] = {
+    glyph = "p",
+    tick = function(entity, potency)
+      entity.hp = entity.hp - potency
+      return string.format("poison saps %s for %d", entity.name, potency)
+    end,
+  },
+  [Status.BURNING] = {
+    glyph = "b",
+    tick = function(entity, potency)
+      entity.hp = entity.hp - potency
+      return string.format("flames sear %s for %d", entity.name, potency)
+    end,
+  },
+  [Status.REGEN] = {
+    glyph = "r",
+    tick = function(entity, potency)
+      local before = entity.hp
+      entity.hp = math.min(entity.maxHp, entity.hp + potency)
+      if entity.hp > before then
+        return string.format("%s knits flesh for %d", entity.name, entity.hp - before)
+      end
+      return nil
+    end,
+  },
+  [Status.STUN] = { glyph = "s", skipsTurn = true },
+  [Status.HASTE] = { glyph = "h", speed = 60 },
+  [Status.SLOW] = { glyph = "l", speed = -50 },
+  [Status.STRENGTH] = { glyph = "S", attack = 3 },
+}
+
+function Status.apply(entity, kind, duration, potency)
+  potency = potency or 1
+  for _, active in ipairs(entity.statuses) do
+    if active.kind == kind then
+      active.duration = math.max(active.duration, duration)
+      active.potency = math.max(active.potency, potency)
+      return active
+    end
+  end
+  local effect = { kind = kind, duration = duration, potency = potency }
+  entity.statuses[#entity.statuses + 1] = effect
+  return effect
+end
+
+function Status.has(entity, kind)
+  for _, active in ipairs(entity.statuses) do
+    if active.kind == kind then return true end
+  end
+  return false
+end
+
+-- Advances every effect one step, applying its tick and dropping expired ones.
+-- Returns the log lines produced this step, in effect order.
+function Status.tick(entity)
+  local lines = {}
+  local kept = {}
+  for _, active in ipairs(entity.statuses) do
+    local def = Status.defs[active.kind]
+    if def.tick then
+      local line = def.tick(entity, active.potency)
+      if line then lines[#lines + 1] = line end
+    end
+    active.duration = active.duration - 1
+    if active.duration > 0 then
+      kept[#kept + 1] = active
+    end
+  end
+  entity.statuses = kept
+  return lines
+end
+
+function Status.speedModifier(entity)
+  local delta = 0
+  for _, active in ipairs(entity.statuses) do
+    local def = Status.defs[active.kind]
+    if def.speed then delta = delta + def.speed end
+  end
+  return delta
+end
+
+function Status.attackModifier(entity)
+  local delta = 0
+  for _, active in ipairs(entity.statuses) do
+    local def = Status.defs[active.kind]
+    if def.attack then delta = delta + def.attack end
+  end
+  return delta
+end
+
+function Status.skipsTurn(entity)
+  for _, active in ipairs(entity.statuses) do
+    if Status.defs[active.kind].skipsTurn then return true end
+  end
+  return false
+end
+
+return Status