Host life-history strategy explains pathogen-induced sterility

Virulence is often equated with pathogen-induced mortality, even though loss of fecundity is also common. But while the former may be understood as a simple consequence of lost host resources for the purposes of pathogen transmission, pathogen-induced sterility is often not associated with changes in host mortality. As a result, a separate literature has emerged to explain fecundity effects of parasitism that has not been integrated into general theories of the evolution of virulence. Here, I present a model of pathogen-induced sterility that is based on the assumption that hosts and pathogens vie for the same host resources for both reproduction and maintenance. Loss of host fecundity can then be explained by the host compensating for its future loss of resources, before infection. Such preinfection "fecundity compensation" may often cause preinfection investment in maintenance to be as low as postinfection levels, despite a loss of total host resources after infection. Thus, sterility is simply explained as a host life-history strategy in a system where the pathogen necessarily steals host resources for its own transmission. In certain circumstances, the pathogen may even be able to manipulate the host to redirect resources away from reproduction and toward maintenance through castration, causing gigantism.