<P> Virulence is a pathogen's or microbe's ability to infect or damage a host . </P> <P> In the context of gene for gene systems, often in plants, virulence refers to a pathogen's ability to infect a resistant host . In most other contexts, 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 . The noun virulence derives from the adjective virulent . Virulent can describe either disease severity or a pathogen's infectivity . The word virulent derives from the Latin word virulentus, meaning "a poisoned wound" or "full of poison ." </P> <P> In an ecological context, virulence can be defined as the host's parasite - induced loss of fitness . 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 . </P>

What is the term used to describe the disease producing power of a microorganism