Diseases take many forms, from the plague to amnesia to magical
afflictions. They can be gained from contact with contagious persons,
receiving infectious bites, experiencing abject terror, being cursed or 
in a variety of other ways. Symptoms vary widely, as well, and each
has some sort of gameplay disadvantage: a coughing fit might wreck
a spellcast, or magical rot might reduce one's constitution. Some 
particularly virulent diseases might be more often or more seriously
symptomatic than others. Some diseases, left untreated, can take a 
character's statistics below zero, with deadly consequences.

It's possible to fight off diseases. The speed of this is affected by 
relevant statistics and racial disease resistance or susceptibility. Races
with a high resistance to a certain type of disease heal much faster,
while those with a high susceptibility heal much slower. Those with
100% susceptibility will not recover from those diseases on their own
at all, regardless of stats, and they must seek outside help to recover.
Some races are immune to certain kinds of disease (magical, mental or
phsyical), which prevents them catching those diseases at all. Others
are disease carriers, which means that they experience no symptoms if
infected with a disease, but others can still catch it from them.

Biomancers can learn to diagnose diseases and cure physical
ailments. Psionicists can train to heal minds. Druidic magic (and 
some advanced biomancers) can cleanse magical diseases. If a disease
is due to a curse, removing the curse might cure the disease as well.
Sikkar and the demon lords have been known to cure their followers 
of all of ailments, at the behest of Templars and Cultists. Finally,
even death does not stop a Gifted One, but no disease survives the
resurrection process.