Host-pathogen interactions: the attributes of virulence

Virulence is one of a number of possible outcomes of host-microbe interaction. As such, microbial virulence is dependent on host factors, as exemplified by the pathogenicity of avirulent microbes in immunocompromised hosts and the lack of pathogenicity of virulent pathogens in immune hosts. Pathogen-centered views of virulence assert that pathogens are distinguished from nonpathogens by their expression of virulence factors. Although this concept appears to apply to certain microbes that cause disease in normal hosts, it does not apply to most microbes that cause disease primarily in immunocompromised hosts. The study of virulence is fraught with the paradox that virulence, despite being a microbial characteristic, can only be expressed in a susceptible host. Thus, the question "What is a pathogen?" begs the question, "What is the outcome of the host-microbe interaction?" We propose that host damage provides a common denominator that translates into the different outcomes of host-microbe interaction.