Living Death (3.5e Disease)
From D&D Wiki
|Infection DC:||18 DC (Injury [Bite])|
|Damage:||1d4 Con and Int each day until one of these stats is less than or equal to 0. Once this happens, you mutate into a zombie like thing.|
The living death is a virus that lowers your Con and Int each day. If your Con runs out first, then you become a zombie. If your Int runs out first, you don't mutate, but you lose control of your mind and will just attack the closest living creature.
You can come into contact with Living Death by either some of the virus entering your blood stream, or if a zombie infected by it bites you.