Virulence refers to the degree of pathogenicity or the ability of a microorganism, such as bacteria, viruses, fungi, or parasites, to cause disease in a host. It describes the severity or harmfulness of the disease produced by the pathogen.

Virulence factors include traits that enable a microorganism to:

Invade and colonize a host. Evade or suppress the host's immune response. Cause tissue damage or dysfunction. In summary, the higher the virulence, the more severe the disease caused by the pathogen.