Many infectious diseases display strong seasonal dynamics. When both hosts and parasites are influenced by seasonal variables, it is unclear if the start of epidemics is limited by host or parasite factors or both. The Daphnia—Pasteuria host—parasite system exhibits seasonal epidemics. We experimentally tested if low Spring temperatures limit the onset of these epidemics. We used sediments from a natural population containing parasite spores at five constant temperatures ranging from 10-20 oC. We added either Daphnia magna resting stages (ephippia), juveniles from largely susceptible clonal cultures or juveniles from largely resistant clonal cultures from the same population. The acceleration of development with increasing temperature was much faster for the parasite than for the host. This finding supports our hypotheses that parasite outbreaks are limited by temperature, and not solely the availability of hosts. These results imply that climate change could lead earlier seasonal epidemics for this host-parasite system.