Parasitism - the interaction between a parasite and its host - is expected to change in a warmer future, but the direction and magnitude of this change is uncertain. One challenge is understanding whether warming effects will be similar on individual hosts (e.g., parasite burden) compared to on population-level parasitism (e.g., prevalence, R0). Examining thirteen empirical systems, we found a strong positive relationship between the thermal optima of individual- and population-level parasitism. We also found that parasitism thermal optima were close to host performance thermal optima in mosquito - parasite systems but not in non-mosquito - parasite systems. A simple mechanistic model showed how population-level parasitism thermal optima can be similar to individual-level parasitism thermal optima even under conditions where parasite transmission has a considerably higher thermal optimum. These results provide a key step towards finding general rules for how warming temperatures should affect parasitism in individuals, populations, and ecosystems.
