In the
RuneQuest RPG, a
shaman may summon many sorts of spirits from
the Spirit plane. Summoning disease spirits is considered "evil" by any reasonable standard; in
Glorantha, only
Chaotic beings do so. The Gloranthan goddess of disease is
Malia, and all the
cult spirits of Malia are disease spirits.
There are several sorts of disease spirits, differentiated by the characteristic they attack. Once the disease spirit has possessed a victim, it reduces the related characteristic at a rate of one point per day. The spirit can be banished by a shaman, or by driving it out with a healing spirit; without such intervention, though, the character will die when the target characteristic is reduced to zero. The disease spirit would then hunt for further victims, or return to the spirit world.
Wasting Disease spirits attack Strength.
Creeping Chills spirits attack Constitution.
Brain Fever spirits attack Intelligence.
Soul Waste spirits attack Power.
Shakes spirits attack Dexterity.
There is no "official" disease spirit associated with Appearance; I suggest some sort of Rot.
Assuming the spirit is driven out before it kills its victim, the characteristic can be restored with the divine "Restore Characteristic" spells.