harmful vs virulent

of a kind likely to be damaging; injurious 

Extremely hostile or malicious; intensely acrimonious. 

Of a pathogen: replicating within its host cell, then immediately causing it to undergo lysis. 

Of a disease or disease-causing agent: malignant, able to cause damage to the host. 

Of animals, plants, or substances: extremely venomous or poisonous.