A game-theoretical model of parasite virulence

The evolution of parasitic reproductive rates, relative infectiousness and severity of disease are considered using a game-theoretical model in which parasites compete within hosts. Each parasite's fitness is assumed to be directly proportional to the product of its reproductive rate (lambda) and the length of time (T) over which it reproduces. An increase in a parasite's reproductive rate is assumed to increase its host's disease-induced mortality rate (alpha) and consequently, through host death, to decrease T. By maximizing the total number of propagules that individuals produce with respect to their individual reproductive rates, we show that competitors within a host may be favored by natural selection to reproduce at rates below their maximum potential rates. Whether competitors behaving with such restraint can coexist at a Nash equilibrium is shown to depend on the functional form of alpha (lambda) and on the number of competitors within a host. While an individual's restraint benefits its within-host competitors through increased host longevity, the model does not invoke group selection. In the model, selection favors an individual's restraint when such behavior increases the individual's total number of propagules. Concurrent increases in the absolute and relative fitness of an individual's within-host competitors can be consequences of such individual selection.