Sickness

Version 0.28: This article may not be up to date for the latest stable release of Crawl.

You have been sickened, and your health will no longer regenerate.

Sickness is a status effect that prevents characters from naturally healing HP until it wears off.

Sources of sickness

Undead and nonliving species and forms (Ghoul, Mummy, bloodless Vampire, Djinn, Statue Form, Storm Form, Necromutation, Shadow Form, Wisp Form) have sickness immunity, as do all undead, nonliving, insubstantial, demonic, and holy monsters.
Certain demonspawn may get sickness immunity as part of the Foul Stench demonspawn mutation.
Quaffing a potion of cancellation cures sickness.
Elyvilon's Purification ability will cure sickness, while Zin's Vitalisation ability provides temporary immunity to sickness.

Prior to 0.28, the status changed colour depending on remaining duration (red -> pink -> yellow).
Prior to 0.27, sickness was cured by drinking a potion of curing instead of a potion of cancellation. Also, increased regeneration rates would cause it to wear off more quickly.
Prior to 0.26, sickness healed slower if hungry.
Prior to 0.19, sickness had a chance to cause a single point of temporary stat damage.
Prior to 0.17, casting Haunt could inflict sickness.
Prior to 0.15, kobolds would recover from sickness more quickly than other species.
Prior to 0.13, sickness was one of the many unique needle brands.
Prior to 0.10, eating contaminated chunks would occasionally cause you to become sick. Saprovores were partially protected from this. In 0.13, nausea was introduced as an intermediate step between health and sickness. Eating food while nauseous would occasionally induce sickness, but had few downsides on its own.