Sickness is a status effect that can be caused by the attacks of poisonous creatures or by eating food that is unsafe. Unsafe food may be caused by age (food degrades over time) or by the inherent properties of the foodstuff, such as the toxicity of kobold innards or toxic mushrooms. The message that precedes sickness gives a general indication of the severity of sickness that will follow.
There are two stages of sickness, mild and severe. Severe lasts longer and does more damage. (confirmation needed)
Sickness causes some damage to the player at irregular intervals. The damage is variable, as is the duration. Sickness will eventually wear off, but will continue to bring the risk of HP loss as long as it remains in play.
Wearing an Amulet of Versus Poison grants immunity to sickness.
Sickness may be cured by: