Pairwise invadability analysis is used to examine the evolutionary dynamics of host resistance to microparasitic infection. A continuum of strains of the host differs in susceptibility to infection, with less susceptible strains paying a cost resulting in a lower intrinsic growth rate. With a combination of analytical and graphical pairwise invadability analysis, we show that the evolutionary outcome depends crucially on the shape of the constraint function between resistance and its assumed cost in intrinsic growth rates. When resistance is increasingly costly, a single evolutionarily stable strategy is predicted. Alternatively, with decreasingly costly resistance, we find that the hosts tend to be maximally resistant or not at all resistant. There are conditions under which dimorphism of both these types exists but intermediate resistances do not occur. Independently of the trade-off function used, we are always more likely to get resistant strains of the host when the carrying capacity of the host is high. The pathogenicity of the parasite is also important in determining the likelihood and degree of resistance.

A fundamental question in both evolutionary biology and parasitology is why do different levels of virulence evolve in different parasites. Here we use explicitly spatial lattice models to show how the spatial relationships of infection and host reproduction determine the degree of virulence that will occur. When the reproduction of the host acts over larger spatial scales than the infection process higher virulence is predicted. In contrast to both the mean‐field and the case where infection acts over larger spatial scales than reproduction, the transmission and virulence predicted are always finite as “self‐shading” of infected individuals always occurs. This process may help to explain the evolution of the high virulence of larval diseases of insects where reproduction clearly acts over greater distances than infection.