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 depended critically on the interaction of exposure and susceptibility; epidemics only emerged when a host population’s level of exposure exceeded its individuals’ capacity for recovery. Additionally, we found that host internal clearance traits (the hemocyte response) were critical in regulating epidemics. Our study provides an empirical demonstration of how parasite exposure and host susceptibility interact to inhibit or drive disease in natural systems and demonstrates that epidemics can be delayed by asynchronicity in the two processes. Finally, our results highlight how individual host traits can scale up to influence broad epidemiological patterns.

All methods related to the present data can be found in the methods and appendix of Stewart Merrill et al. 2020 Ecology. 

Parasite_data.xlsx contains the raw exposure data, including each individual Daphnia's infection stage and the number of spores they were exposed to.

Immune_data.xlsx contains the raw susceptibility data for wildcaught Daphnia exposed experimentally to Metschnikowia in the lab.

Parasite_data_summarized.xlsx summarizes infection stages (to provide prevalences), while also providing average exposure (spore counts) and susceptibility (susceptibility thresholds) for each lake by time combination.

See metadata tabs in each data file for information on usage. Metadata describes each column header, provides supplementary information, and lists any exclusions that may be necessary for analyses.