virulent

Short definition from Wiktionary

Adjective

1. 1
   Of animals, plants, or substances: extremely venomous or poisonous.
2. 2
   Extremely hostile or malicious; intensely acrimonious.
3. 3
   Of a disease or disease-causing agent: malignant, able to cause damage to the host.
4. 4
   Of a pathogen: replicating within its host cell, then immediately causing it to undergo lysis.