An Illness is a type of Condition. They represent medical afflictions that can harm the player.
There are three different types of Illnesses: Diseased, Poisoned, and Infection. All three of them prevent Health recovery during a Rest Action, while Poisoned also prevents Sanity recovery. During the rest action, Infections can be discarded by rolling a 4, 5, or 6, while Diseased can be removed on a 5 or 6. Both Infection and Diseased require a roll that flips the card on a failure. Some of the Conditions allow a discard after applying the negative effects, and others merely flip the card over.
Poisoned triggers on a Rest Action, and allow for a skill test. If the player passes, they can discard the card. If they fail, they cannot, suffer a negative effect, and the card flips back over.