Within-Host Population Dynamics and the Evolution and Maintenance of Microparasite Virulence

Abstract

We examine effects of the vertebrate immune system on the evolution and maintenance of virulence of microparasites (viruses, bacteria, or unicellular eukaryotes). We employ a mathematical model of the within-host population dynamics of an invading microparasite and the immune response. In this model, the virulence of the parasite is proportional to its rate of replication within the host. We show that if the rate of transmission of parasites from infected hosts is proportional to their within-host density, then the total transmission reaches a maximum for parasites with intermediate rather than low or high virulence. When hosts can only be infected with a single parasite strain, then parasites having intermediate virulence not only release the most transmissible forms from individual infected hosts but also drive other parasite strains to extinction in the population at large. We interpret the results of our analysis of the properties of this model as support for the proposition that the immune system may be responsible for the evolution and maintenance of the virulence of those microparasites that it controls and clears.