Virulent

Virulent: Extremely noxious, damaging, deleterious, disease-causing (pathogenic). Marked by a rapid, severe, and malignant course. Poisonous, venomous.

The virulence of a microorganism is a measure of the severity of the disease it causes.

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