Virulence is a pathogen's or microorganism's ability to cause damage to a host.

In most, especially in animal systems, virulence refers to the degree of damage caused by a microbe to its host. The pathogenicity of an organism—its ability to cause disease—is determined by its virulence factors. In the specific context of gene for gene systems, often in plants, virulence refers to a pathogen's ability to infect a resistant host.

The noun virulence derives from the adjective virulent, meaning disease severity. The word virulent derives from the Latin word virulentus, meaning "a poisoned wound" or "full of poison."

From an ecological standpoint, virulence is the loss of fitness induced by a parasite upon its host. Virulence can be understood in terms of proximate causes—those specific traits of the pathogen that help make the host ill—and ultimate causes—the evolutionary pressures that lead to virulent traits occurring in a pathogen strain.