Virulence is the degree of pathogenicity of an organism: the relative ability of a pathogen to cause disease. The noun virulence derives from the adjective virulent. In discussing disease, the adjective virulent is used to describe effect severity, and in discussing pathogens, the degree of infectiousness.Compact Oxford English Dictionary virulent The word virulent derives from the Latin word virulentus, meaning "a poisoned wound" or "full of poison." In an ecological context, virulence can be… (More on Virulence)