Host-pathogen interactions: The attributes of virulence

Virulence is one of a number of possible outcomes of host-microbe interacti on. As such, microbial virulence is dependent on host factors, as exemplifi ed by the pathogenicity of avirulent microbes in immunocompromised hosts an d the lack of pathogenicity of virulent pathogens in immune hosts. Pathogen -centered views of virulence assert that pathogens are distinguished from n onpathogens by their expression of virulence factors. Although this concept appears to apply to certain microbes that cause disease in normal hosts, i t does not apply to most microbes that cause disease primarily in immunocom promised hosts. The study of virulence is fraught with the paradox that vir ulence, despite being a microbial characteristic, can only be expressed in a susceptible host. Thus, the question "What is a pathogen?" begs the quest ion, "What is the outcome of the host-microbe interaction?" We propose that host damage provides a common denominator that translates into the differe nt outcomes of host-microbe interaction.