vir·u·lent 
(vîryə-lənt, vîrə-)
(vîryə-lənt, vîrə-)adj.
1.
a. Characterized by, causing, or promoting the rapid onset of severe illness. Used of a disease or toxin.
b. Capable of causing disease by aggressively interfering with the immune system of the host. Used of a pathogen.
2. Extremely hostile or malicious: virulent criticism; a virulent bigot.
[Middle English, from Latin vīrulentus, from vīrus, poison.]
viru·lence, viru·len·cy n.
viru·lent·ly adv.