New insights into virulence evolution in multigroup hosts.

Many of the standard predictions in evolutionary epidemiology result from models in which all hosts are equally susceptible to acquiring an infection and equally capable of resisting pathogens once an infection has been established. This contrasts with the empirical reality that natural host populations are typically composed of individuals with various susceptibilities and vulnerabilities to pathogen exploitation that can influence all aspects of a given pathogen's transmission-virulence phenotype. In these structured host settings, host-dependent variation in the virulence-transmission trade-off plays an important role in determining pathogen evolution. By deriving some game-theoretic equilibrium expressions that describe pathogen evolution in heterogeneous host populations, the contribution of host heterogeneity to the direction of evolution in host exploitation is made explicit. Within this framework, qualitative departures from predictions derived from theory utilizing a homogeneous host assumption can be seen as a manifestation of Simpson's paradox in an evolutionary setting. By reconsidering some predictions from homogeneous host theory through the lens of this new perspective, it can be seen that many standard predictions are actually special cases that result when homogeneity in immunity parameters is imposed on host populations.