Virulence

Virulence: The ability of any agent of infection to produce disease. The virulence of a microorganism (such as a bacterium or virus) is a measure of the severity of the disease it is capable of causing.

The adjective virulent implies extremely noxious, damaging, deleterious, disease-causing (pathogenic). Marked by a rapid, severe, and malignant course. Poisonous, venomous.

The word "virulence" comes from the Latin "virulentia" from "virus" meaning a slimy liquid, particularly one that is foul and poisonous.