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.