Parasite prevalence shows tremendous spatiotemporal variation. Theory indicates this variation might stem from life history characteristics of parasites and key ecological factors. Here, we illustrate how the interaction of an important predator and the schedule of 'transmission potential' of two parasites can explain parasite abundance. A field survey showed that a non-castrating fungus (Metschnikowia bicuspidata) commonly infected a dominant zooplankton host (Daphnia dentifera), while a castrating bacterial parasite (Pasteuria ramosa) was rare. This result seemed surprising given that the bacterium produces many more infectious propagules (spores) than the fungus upon host death. The fungus's dominance can be explained by the schedule of within host growth of parasites (i.e., how transmission potential changes over the course of infection) and spore release from 'sloppy predators' (Chaoborus spp., who consume Daphnia prey whole, then later regurgitate the carapace and parasite spores). In essence, sloppy predators create a niche that the faster-schedule fungus currently occupies. However, a selection experiment showed the slower-schedule bacterium can evolve into this faster-schedule, predator-mediated niche (but pays a cost in maximal spore yield to do so). Hence, our study shows how parasite life history can interact with predation to strongly influence the ecology, epidemiology, and evolution of infectious disease. 18 pages, 1 table, 5 figures; Appendix