Corpseblight (5e Disease)
From D&D Wiki
This fungal infection can be carried by a mundane living creature, but won't have any noticeable effect. However, it can infect undead; even those that have disease immunity. An undead coming into contact with the disease must make a Constitution saving throw with a DC of 15 or be infected. Any dead or undead body the fungus has infected will cause it to quickly decay. Every hour, the afflicted must make another Constitution saving throw or take 1d6 + 6 necrotic damage. This damage ignores damage resistance and immunity. Upon reaching 0 hit points, the afflicted disintegrates entirely. After three successful saves, the disease is warded off. Any already-dead corpse infected with the disease will disintegrate in 24 hours, leaving no trace.
Strains of this disease are carried in vials by some clerics, specifically to repel hordes of undead that could otherwise overwhelm a town, but their usage of this disease is a closely guarded secret.