Sickness may be caused by:
- Drinking dirty water or vomit
- Using cursed healing rods or potions ("you feel grumpy. you get sick.")
- Eating certain kinds of food (Cursed corpses of undead: Zombie, Skeleton, etc)
- Drinking alcoholic Potions
Sickness may be cured if you drink a blessed healing potion (of any kind), or when you sleep; however, the only guaranteed cure is if you die and crawl back up. Dying in the arena/Party Time!/Hunting quests can be used to instantly cure your sickness, however, it can incur penalties to fame/karma (unless you manage to kill all monsters first and then somehow die without leaving: starvation, overweight, summoned monsters, overcasting, etc), as well as receive the Tired status.
The stat loss can be avoided by wearing "It maintains ..." equipment. Wearing a full set of it will make you immune to sickness.
Drinking green tea will cure sickness. (Version 1.68)