Virulence is a harmful quality possessed by microorganisms that can cause disease. You can also use the noun virulence to describe someone's malicious actions.
A virus spreads disease, and virulence is a quality possessed by viruses that are on the loose and spreading. This could be as common as the flu or as unusual as chemical warfare. You can also speak of the virulence of hostile, harmful words and actions. Publicly insulting and discrediting someone is an example of virulence. With either meaning, virulence indicates danger and harm. The Latin root is virulentus, "poisonous."