Summary1. Predators could reduce disease prevalence in prey populations by culling infected hosts and reducing host density. However, recently observed positive correlations between predator density and disease burdens in prey ⁄ hosts suggest that predators do not always 'keep the herds healthy'. Several possible mechanisms could explain this 'unhealthy herds' effect, including a predator-induced change in prey ⁄ host traits which enhances susceptibility or alters other epidemiologically important traits. 2. Here, we use an invertebrate predator, zooplankton host, yeast parasite system to demonstrate such trait-mediated indirect effects. We exposed ten genotypes of the prey ⁄ host Daphnia dentifera to infochemicals ('kairomones') produced by the invertebrate predator Chaoborus and to a yeast parasite. 3. We found that kairomone exposure induced larger and more susceptible D. dentifera. Clones that showed substantial increases in body length also yielded more spores upon death. However, exposure to kairomones did not alter reproduction from uninfected hosts. All of these results were captured with a dynamic energy budget model of parasitism. 4. Overall, our empirical and theoretical results show that predators can have strong indirect effects on host-parasite interactions that could produce positive correlations between predation intensity and disease burden.

1. Recently, the potential for parasites to influence the ecology and evolution of their zooplankton hosts has been the subject of increasing study. However, most research to date has focussed on Daphnia hosts, and the potential for parasites to influence other zooplankton taxa remains largely unstudied. 2. During routine sampling of zooplankton in a eutrophic lake, we observed that the rotifer Asplanchna girodi was often infected with a parasitic oomycete. Epidemics of this parasite occurred frequently, with three separate events in a single year. Prevalence at peak infection ranged from 29 to 41% and epidemics lasted from 17 to 56 days. Our data indicate that high densities of the host population are required for epidemics to occur. 3. Our morphological and molecular analyses suggest that this parasite is in the genus Pythium. Most Pythium spp. are plant pathogens, but our study supports recent work on Daphnia, suggesting that Pythium spp. are also important parasites of zooplankton. 4. As the parasite in this study was recalcitrant to cultivation, we developed an alternative method to verify its identity. Our approach used quantitative PCR to show that the ribosomal sequences identified increased with increasing density of infected hosts and, thus, were associated with the parasite. This approach should be generally applicable to other plankton parasites that are difficult to cultivate outside their hosts. 5. Infections significantly reduced host fecundity, lifespan and population growth rate. As a result of the virulence of this parasite, it is likely to influence the population ecology and evolution of its Asplanchna host, and may be a useful model system for studies on hostparasite coevolutionary dynamics.