Demon fever

Demon fever was a disease spread by night hags and abyssal ghouls, neither of which, strangely, were demons.

Acquisition
The disease was transmitted via the bite of a night hag and via the front and hind claws of an abyssal ghoul. It had an incubation period of one day, and was hard to resist.

Effects
Each day of infection, it could greatly damage the sufferer's health and vitality, making it harder for them to fight off the disease. If the disease was healed or resisted, this health loss could be recovered. However, demon fever had a secondary effect: if not resisted, it also drained health and vitality permanently, though to a lesser degree. This continued until the victim died.