Each turn, two random cities of size 5 or larger may suffer a DISASTER. The kind is rolled at random and only fires if its local trigger is met:
EARTHQUAKE — Hills nearby
PLAGUE — no AQUEDUCT
FLOODING — River, no Walls
VOLCANO — Mountains, no Temple
FAMINE — no GRANARY
FIRE — no AQUEDUCT
PIRATES — Ocean, no Barracks
FEVER — Jungle, no Medicine
RIOT — unhappy > happy
Each disaster has its own effect. Most kill 1/4 to 1/3 of the city's size; EARTHQUAKE and FIRE instead destroy a random building (never the Palace); PIRATES zero food and shields for the turn; RIOT can let a happier city annex yours.
FEVER spares the Olvir and any jungle tile with a Canopy Array.
HURRICANES are tracked separately — see the HURRICANE entry for the latitude-band rules and the SEA PLATFORM mitigation.