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Epidemiological models generally assume that the number of susceptible individuals that become infected within a unit of time depends on the density of the hosts and the concentration of parasites (i.e. mass-action principle). However, empirical studies have found signi?cant deviations from this assumption due to biotic and abiotic factors, such as seasonality, the spatial structure of the host population and host heterogeneity with respect to immunity and susceptibility. In this paper, we examine the effect of the dose level of the bacterial endoparasite Past e ur i a r amos a on the infection rate of its host, the water ?ea Daphnia magna . Using seven host clones and two parasite isolates, we measure the fraction of infected hosts after exposure to eight different parasite doses to determine whether there is variation in the infection process across different host clone?parasite isolate combinations. In ?ve combinations, a pronounced dose-dependent infection pattern was found. Using a likelihood approach, we compare the infection data of these ?ve combinations to the ?t of three mathematical models: a mass-action model, a parasite antagonism model (i.e. an increase in the parasite dose leads to an under-proportionate increase in the infection rate per host) and a heterogeneous host model. We found that the host heterogeneity model, in which we assumed the existence of non-inherited phenotypic differences in host susceptibilities to the parasite, provides the best ?t. Our analysis suggests that among 5 out of the 14 host clone?parasite isolate combinations that resulted in appreciable infections, non-genetic host heterogeneity plays an important role. Keywords: D aphni a magn a ; dose?response curve; frailty model; mass-action infection; Past e ur i a r amos a
