Black Fly Filth Fever

Black Fly filth fever is a disease spread by the various species of black fly due to their proximity to decaying matter and bodily waste. It is contracted through injury, and causes the victim's wounds to fester and rot and their skin erupts in small but painful boils.
Once contracted, the victim becomes poisoned until the disease is cured. While thus poisoned, the victim cannot regain HP except by magical means, and the target's HP maximum decreases by 3 (1d6) every 24 hours. If the target's hit point maximum drops to 0 as a result of this disease, the target dies.