The role of spatial population structure on the evolution of parasites with acquired immunity and demography.

It is clear that the evolution of infectious disease may be influenced by population spatial structure and transmission networks but we lack an understanding of the role of acquired immunity. Here we examine theoretically the role of spatial structure in the evolution of infectious disease described by the classic Susceptible, Infected, Recovered (SIR) model focusing on the impact of host demographics. We find that, for the classic assumption of a trade-off between transmission and virulence, localised transmission does favor, as predicted from other models, chronic pathogens with low transmission and virulence, but that this effect reduces as the recovery rate increases. However, under the assumption that pathogens reproduce rapidly within the host are harder to clear but result in higher virulence local interactions favor more virulent parasites and, depending on the nature of the disease interaction, can increase or decrease the chance of evolutionary bistabilities that may lead to sudden persistent changes in virulence. Therefore, our work further emphasizes the importance of spatial structure to parasite evolution. This spatial evolutionary theory is important because it predicts how different pathogens may respond to changes in patterns of mixing.