Variable, changing, climates may affect each participant in a biotic interaction differently. We explored the effects of temperature and plasticity on the outcome of a host-pathogen interaction to try to predict the outcomes of infection under fluctuating temperatures. We infected Gryllus veletis crickets with the entomopathogenic fungus Metarhizium brunneum under constant (6 °C, 12 °C, 18 °C or 25 °C) or fluctuating temperatures (6 °C to 18 °C or 6 °C to 25 °C). We also acclimated crickets and fungi to constant or fluctuating conditions. Crickets acclimated to fluctuating conditions survived best under constant conditions if paired with warm-acclimated fungus. Overall, matches and mismatches in thermal performance, driven by acclimation, determined host survival. Mismatched performance also determined differences in survival under different fluctuating thermal regimes: crickets survived best when fluctuating temperatures favoured their performance (6 °C to 25 °C), compared to fluctuations that favoured fungus performance (6 °C to 18 °C). Thus, we could predict the outcome of infection under fluctuating temperatures by averaging relative host-pathogen performance under constant temperatures, suggesting that it may be possible to predict responses to fluctuating temperatures for at least some biotic interactions.