Evolution of virulence in a host-pathogen system with local pathogen transmission

If hosts can be effectively infected by more than a single pathogen and pathogen transmission is spatially homogeneous, natural selection favors higher virulence levels, since pathogens run the risk to share their host with other co-exploiters or lose it entirely to a stronger competitor. In a host-pathogen system where pathogen transmission is localized between neighboring hosts, infected hosts either occur in relatively small patches, isolated from each other by susceptible hosts, or the entire spatial domain is more or less evenly covered with infected hosts with only few susceptible hosts in between. The patchy distribution arises when multiple pathogen infections have a large negative effect on pathogen productivity and leads to less virulent pathogens being favored. The contiguous distribution of infected hosts occurs when multiple infections are less deleterious and leads to more virulent pathogens being favored. In contrast, when multiple host infections are not possible, selection favors the pathogen with a maximal basic reproduction ratio irrespective of the mode of pathogen transmission. The spatially heterogeneous distributions that emerge because of local pathogen transmission have additional consequences for pathogen invasion and population dynamics.