The frequency and severity of heatwaves are increasing, posing challenges for understanding their effects on host-parasite dynamics. Especially, our understanding of the role of specific heatwave attributes in shaping disease outcomes remains limited. In this study, the Daphnia magna–Ordospora colligata host-parasite system, a widely used model for environmentally transmitted diseases, was used to investigate heatwave attributes. The amplitude and duration of heatwaves were manipulated across four baseline temperatures and four distinct time points relative to host exposure to the pathogen. This design resulted in 64 unique heatwave treatments, with O. colligata fitness (measured as prevalence and proliferation) recorded at the individual level in temperature-controlled water baths. Results show that heatwaves can alter parasite burden up to 13-fold, whereby amplitude, duration, and timing can interact with baseline temperature. Our results reveal complex interactions between heatwave attributes and baseline temperature, emphasising that heatwaves have context-dependent effects on parasite prevalence and proliferation. Additionally, when compared to other types of temperature variation (for example, cold snaps), heatwaves behave differently. While specific effects may vary across systems, these results demonstrate that interactions between heatwave attributes and baseline temperature can drive substantial variation in infection outcomes. These findings highlight the challenges and complexities involved in understanding and predicting how climate change and extreme weather events may influence disease dynamics in the context of global change. This underscores the need to incorporate thermal fluctuations into disease ecology models, as host-parasite responses to climate extremes are unlikely to be uniform across taxa.