The evolution of pathogen virulence: Effects of transitions between host types

Much of evolutionary epidemiology theory is derived from a perspective in which all hosts, and all parasites, are epidemiologically equivalent. This stands in contrast to the well-documented existence of the numerous processes generating heterogeneity among hosts and parasites that can profoundly influence evolutionary/epidemiological dynamics. Age-related immunological changes, inequities in nutritional status, and interactions between parasites via coinfection are just a few of the many factors that generate heterogeneity among hosts in the ways they express susceptibility to, and respond to infection by, a focal pathogen. Moreover, organisms age, nutritional states improve or worsen, and co-infections can be cleared or acquired, implying that transitions between these different disease states are the rule, rather than the exception, in natural disease systems. Here we develop the theoretical framework for modeling the implications of such transitions in these multi-type host settings for the evolution of virulence. Results show that ignoring these common sources of host heterogeneities in disease characteristics can lead to both quantitatively and qualitatively mischaracterized evolutionary predictions.