SIMPLE mathematical models for microparasites offer a useful way to examine the population dynamics of different viral and bacterial pathogens, One constraint in applying these models in free-living host populations is the paucity of data with which to estimate transmission rates. Here we recast a standard epidemiological model by setting the birth and death rates of the host population and its density as simple allometric functions of host body weight. We then use standard threshold theorems for the model in order to estimate the minimum rate of transmission for the parasite to establish itself in a mammalian host population, Transmission rates that produce different comparable values of the parasites’ basic reproductive number, R(o), are themselves allometric functions of host body size. We have extended the model to show that hosts having different body sizes suffer epidemic outbreaks whose frequency scales,vith body size. The expected epidemic periods for pathogens in different mammalian populations correspond to cycles observed in free-living populations.