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is the ability of an agent of infection to produce disease. Hence, virulence is the degree of damage caused by a microbe to its host. Ie, their ability to cause disease. The pathogenicity of an organism - its ability to cause disease - is determined by its virulence factors. "Virulence" derives from "virulent" and can describe either disease severity or a pathogen's infectivity. The ability of bacteria to cause diseases is described in terms of the number of infecting bacteria, the route of entry into the body, the effects of host defense mechanisms, and intrinsic characteristics of the bacteria called virulence factors. Many virulence factors are so called "effector proteins" that are injected into the host cells by special secretion "machines", or rather systems such as the Type III Secretion System (see above).