Capable of causing damage or dysfunction by toxicity; (broadly) poisonous.
A toxic or poisonous substance.
Of animals, plants, or substances: extremely venomous or poisonous.
Of a pathogen: replicating within its host cell, then immediately causing it to undergo lysis.
Extremely hostile or malicious; intensely acrimonious.
Of a disease or disease-causing agent: malignant, able to cause damage to the host.