Human disease and the evolution of pathogen virulence.

Theorists who make a priori generalizations about the tendency of host-parasite systems to co-evolve toward commensalism or increased parasitism err because they do not consider the empirical relation between host pathology and pathogen transmission. Natural selection should favor increased pathogen virulence in diseases where host pathology contributes to pathogen transmission and favor decreased virulence when pathology impedes transmission. This paper classifies important human diseases according to the contribution human pathology makes to pathogen transmission.