What is a pathogen?

The question 'what is a pathogen?' derives from pathogen-centered views of microbial pathogenesis that invoke microbial characteristics to explain virulence. However, virulence cannot be an independent microbial variable since it is only expressed in a susceptible host. Hence, the question 'what is a pathogen?' cannot be separated from the question 'what is a host?' Given that virulence is an outcome of the host-microbe interaction that is not invariant from host to host, neither the pathogen or the host can be defined independently. To sidestep this paradox, one can focus on the outcome of the interaction using host damage as the common denominator to define and explain the various outcomes that can result from the host-microbe interaction. Using this concept a pathogen is defined as a microbe capable of causing host damage.