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.