Parasitism 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 parasitism within or on individual hosts (e.g., parasite load) compared to on population-level parasitism (e.g., prevalence, R0). We adapted a simple temperature-dependent model and simulated several scenarios for individual- and population-level parasitism. Our model found that small differences in the underlying biology of host-parasite systems can substantially alter the expected relationship between the thermal optima of parasitism across levels of organization. In thirteen empirical host-parasite systems, we found a strong positive correlation between the thermal optima of individual- and population-level parasitism, suggesting that the effects of warming on parasitism may often be in the same direction across levels. 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. Generally, our results suggest that information on the temperature-dependence, and specifically the thermal optima, of a host-parasite system at either the individual- or population-level should provide a useful--though not quantitatively exact--baseline for predicting temperature-dependence at the other level in a variety of host-parasite systems.
