Parasite exposure and host susceptibility jointly drive the emergence of epidemics

Parasite transmission is thought to depend on both parasite exposure and host susceptibility to infection; however, the relative contribution of these two factors to epidemics remains unclear. We used interactions between an aquatic host and its fungal parasite to evaluate how parasite exposure and host susceptibility interact to drive epidemics. In six lakes, we tracked the following factors from pre-epidemic to epidemic emergence: 1) parasite exposure (measured observationally as fungal spores attacking wild-caught hosts), 2) host susceptibility (measured experimentally as the number of fungal spores required to produce terminal infection), 3) host susceptibility traits (barrier resistance and internal clearance, both quantified with experimental assays), and 4) parasite prevalence (measured observationally from wild-caught hosts). Tracking these factors over six months and in almost 7,000 wild-caught hosts provided key information on the drivers of epidemics. We found that epidemics ...