Virulence Evolution in Host Populations with Heterogeneous Immune Responses: The Weakening of the Uncommon Weak

How and to what extent different factors in pathogen-host dynamics shape virulence has been largely debated due to evolutionary forces acting in opposite directions. Here we present a model of pathogen-host dynamics with the aim of studying how competition between pathogens and the immune response of the host affect virulence. The model describes, and links, the pathogen dynamics inside individual hosts and the dynamics of the susceptible and infected individuals in the host population. We first show that when the hosts have an adequate immune response (so that they completely recover from the infection on a short time scale), pathogens tend to increase their virulence. On the contrary, if the hosts are vulnerable (meaning that they will eventually die from the infection), low virulences will be selected. We then move to a richer scenario in which the host population is mixed and contains a fraction p of vulnerable individuals. We see that the selected virulence exhibits biologically relevant behaviors as p takes intermediate values.