Climate change is increasing the frequency and intensity of temperature extremes, yet the consequences of heatwaves and cold snaps for host–parasite interactions remain incompletely understood. Thermal pulses of ±6 °C were experimentally tested to determine how disease dynamics are affected in populations of Daphnia magna infected with Ordospora colligata across 4 baseline temperatures (14, 17, 20 and 23 °C). Both heatwave and cold snap effects were strongly temperature-dependent and reflected how far temperature deviations shifted the system towards or away from the parasite’s thermal optimum. At 14 °C, heatwaves produced short-term increases in parasite burden, whereas no such effect occurred at 17 °C. Cold snaps also generated short-term increases in parasite fitness at warmer baselines (23 °C). Host population size also varied with baseline temperature and treatment, and parasite burden was linked to host density: burden and density effects uncoupled at the warmest baseline. These results show that extreme temperature variation can reshape both demographic and infection dynamics in complex, context-dependent ways. As climate extremes intensify, incorporating short-term thermal variability and its interactions with baseline conditions will be essential for predicting and managing disease risk in natural freshwater systems.