Large shifts in pathogen virulence relate to host population structure.

Theory on the evolution of virulence generally predicts selection for an optimal level of virulence determined by trade-offs with transmission and/or recovery. Here we consider the evolution of pathogen virulence in hosts who acquire long-lived immunity and live in a spatially structured population. We show theoretically that large shifts in virulence may occur in pathogen populations as a result of a bistability in evolutionary dynamics caused by the local contact or social population structure of the host. This model provides an explanation for the rapid emergence of the highly virulent strains of rabbit hemorrhagic disease virus.