Understanding pathogens in the era of next generation sequencing.

What is a pathogen? Medical textbooks usually define a pathogen as any microorganism that causes disease. However, this widespread definition is problematic on a number of counts [1, 11]. Moreover, a generally accepted definition is not forthcoming among medical microbiologists, immunologists, and physicians. Here it is argued that there is another, and more pressing question to be asked, namely: what makes some organisms pathogenic and others not? Asking these questions instead allows for distinguishing pathogens from non-pathogens in a more flexible way, while at the same time emphasizing the roles of ecological and evolutionary processes in determining pathogenicity in infectious diseases.