Virulence refers to the degree of pathogenicity of a microbe, or in other words the relative ability of a microbe to cause disease.The word virulent, which is the adjective for virulence, derives from the Latin word virulentus, which means "full of poison." From an ecological point of view, virulence can be defined as the host's parasite-induced loss of fitness. Virulent bacteria The ability of bacteria to cause disease is described in terms of the number of infecting bacteria, the rou