Virulent damage is one of the 8 damage types in the game. It includes damage from "poisons, toxins, and invasive microorganisms".
Virulent Damage is usually considered 'direct damage'. All lifeforms that are not immune to Virulent Damage take damage from Virulent directly to Health, bypassing Armor. Virulent damage also has the special property of always being 'damage over time', no matter the source. The unfortunate downside to Virulent damage is Virulent Weapons cause the least amount of damage in Tabula Rasa.
Virulent critical hits will cause a snare. [Deployment 8]
In addition, all armored creatures have a variable virulent resistance depending on how much armor they have remaining. A creature with full armor will only take 75% damage from any Virulent weapon. This resistance is applied before normal resistance, and they do not stack.
For more detailed information about the effects of damage, see: damage type