Virulence is a monsters-only spell which strips its target of their poison resistance by inflicting the rPois- status effect. It also increases the current poison level of its victim to 150% of its current level. This has little effect when cast on a lightly poisoned character, but can lead to a massive increase in damage taken for characters already at red or dark red poison levels. The spell must overcome your magic resistance to affect you.
Remember that no matter how much poison you have coursing through your veins, one potion of curing will remove all of it.
The following monsters cast Virulence:
Virulence was added in 0.14.