Infections carry inherent tradeoffs in growing hosts due to the changing demands for resource allocation among key processes (i.e., growth and immunity). We implemented dynamic state variable models to determine which changes in energy allocation maximize fitness in amphibians with enzootic infections. By accounting for seasonality in our mechanistic models, we identified critical windows that maximize individual growth while limiting mortality under increased pathogen burden. The model predicted that seasonality in pathogen exposure and foraging success exacerbate growth-defense tradeoffs, resulting in delayed maturity and lower survival when frogs hatched under sub-optimal environmental conditions. Our simulations support well-known field patterns showing that increased frog reproduction coincides with high resource availability and low pathogen risk. Our models can be further parameterized to understand the effect of emerging diseases under predicted climate change across taxa, and to evaluate the best timing for species re-introductions that would reduce fitness tradeoffs in the population.