Nurgle's Rot

Though any disease can drive a mortal into the waiting arms of the Lord of Decay, Neiglish Rot is the most certain. This virulent disease wreaks havoc with the system. Painful buboes erupt in the soft tissues, and it’s always accompanied by a high fever. Many sufferers of this terrible disease slip into madness, or at the very least, commit suicide to avoid experiencing its nastiest effects. Neiglish Rot kills slowly, sometimes taking weeks for the victim to succumb to its ravages. What’s worse is that mutation almost always too, so that even those victims who manage to recover can look forward to a fiery death strapped to a Witch Hunter’s stake. Worst case scenario, they may even transform into Plaguebearers.

Neiglish Rot spreads through contact. The infected merely has to touch another to spread its taint. Occasionally, this plague pops up on its own, but individuals can catch it in a variety of other ways, including being struck by an infected Champion of Nurgle, stepping in a sticky pool left by a Great Unclean One, or by touching a Death Head. Such exposures are all blessedly rare.

Source

 * Warhammer Fantasy RPG 2nd ED -- Tome of Corruption (pg. 19).