Creeping Oak (5e Disease)
From D&D Wiki
Imagine you kick down a door, and get a small splinter. No big deal, right? Wrong, because you might have been infected with Creeping Oak, a disease that, if you don't succeed a DC 15 Constitution Saving Throw, and if not cured within a week, will turn you into a tree. These trees look like any normal tree, except their sap is red, and they smell of meat when burned. Infection spreads from splinters that come from people already transformed. The origin of the illness is a druid who cursed his enemy, not knowing that curse would spread.