virulent

Concise Oxford English Dictionary © 2008 Oxford University Press:
virulent /ˈvɪrʊl(ə)nt, ˈvɪrjʊ-/

▶adjective

• 1 (of a disease or poison) extremely severe or harmful in its effects.

  ■ (of a pathogen, especially a virus) highly infective.

• 2 bitterly hostile.

– derivatives
virulence noun,
virulently adverb.

– origin ME (orig. describing a poisoned wound): from L. virulentus, from virus (see virus).