noun. Same as virulence.

noun. Dated form of virulence.

noun. extreme harmfulness (as the capacity of a microorganism to cause disease)

noun. extreme hostility

"The fever, though it was October, had scarcely abated; indeed, on the contrary, it seemed to have revived and increased in virulency in consequence of the premature return of many people who had fled on its first appearance, and who in coming back too soon to the infected atmosphere, were less able to withstand contagion than those who had remained."

harmfulness  hostility  ill will  injuriousness