Diseases are temporary status effects that can be contracted by the player. Unlike mutations, they provide no positive effects and only negative ones. Diseases can be either cured by using a Disease Cure or waiting until they expire.
Diseases are contracted by:
Sleeping on the ground in bedrolls or on mattresses.
Coming into contact with diseased critters.
Swimming in contaminated, irradiated waters.
Being exposed to toxic air (e.g. in The Mire) without a gas mask on.