Disease is a common affliction caused by injuries, prolonged starvation, certain magic sites and several other sources. It can be the bane of many valuable units, as many nations lack the means to effectively combat it.
Each turn, when most units heal to their max HP, a diseased unit instead takes one tenth of their HP in damage. This means, barring your intervention, most units will die after 10 turns of being diseased. Being injured in combat obviously hastens this demise.
(A diseased unit with Regeneration does NOT lose 10% of their life each turn, but nor do they heal - their HP remains static outside of battle. Notably, a diseased, regenerating unit who is sent into battle WILL regenerate any HP lost as normal, up to their max HP. Thus, if a valuable unit becomes diseased, giving them regeneration in battle will heal them back to max, delaying their death, and giving them regeneration outside of battle will greatly increase their lifespan.)
In addition to the damage over time, Disease also causes the afflicted unit to slowly accrue more afflictions in addition to the disease. This affects even regenerating units, though they get the stardard 75% chance to avoid afflictions due to their regeneration.
Diseases can be cured with a 100% success rate by both Disease Healers and normal Healers.
Undead are immune to the damage dealt by a Disease, though it is possible they may recieve some additional afflictions from it.
Demons are completely immune to disease.