Animals can adopt various behavioural strategies to manage rising pathogen risks under climate change. Nutrition is a key ecological factor in sustaining complex behavioural responses. However, the impact of nutrient availability on behavioural thermoregulation remains poorly understood. Using a Drosophila-fungi system, we tested whether direct nutritional intervention alleviates the physiological costs of infection-related thermoregulatory behaviour. We show that infected insects employ cold-seeking behaviour to enhance survival, but this strategy is only effective when hosts access specific nutrients that offset its reproductive penalties. Yeast in cooler environments maintains a positive lifespan-reproduction correlation, while fungal virulence decreases when hosts consume yeast or amino acids like tryptophan, during the critical window preceding infection intensification. Additionally, tryptophan provision alone is as effective as cold-seeking in reducing microbe loads. These findings highlight that behavioural thermoregulation depends on nutrient availability and composition, providing a mechanistic framework for understanding behavioural immunity under climate change.