A spell of some form
Smallpox, formerly commonly known as simply the pox, is a fatal disease that presents as a serious rash and series of blisters that cover the whole body. Though considered incurable outside of vaccination in the Muggle world, wizardkind can cure the pox with a spell. In his life, the Fat Friar cured many peasants of the pox with said spell. This, along with his habit of conjuring rabbits from the communion cup, caused his senior churchmen to become suspicious of him, and he was executed.
- Pottermore (Mentioned only)