Global warming and temperature variation are likely to have profound impacts on fish as ectotherms that are heavily reliant on environmental temperature for growth, development, metabolism and immunity. This study addresses the impact of climate change on the development of infection and immunity in three-spined sticklebacks (Gasterosteus aculeatus) and their common parasites. In addition to thermal consequences on host parasite interactions, the study also addressed the effects of co-infection on parasite intensities and pathology on host swimming ability. Experiments were designed to mimic global warming, temperature variability and stochasticity (Chapters 3-5). Temperature during exposure to Saprolegnia parasitica was the major determinant of high infection prevalence and intensity with historical temperature exposure having little impact (Chapters 3-5). A further contributor to infection risk was higher host body condition (Chapter 3 and 5), attributed to a trade-off between host immunity and condition, higher condition individuals investing less in immunity supported by a decline in β-def expression in high condition fish (Chapter 5). Peak infection intensities in Gyrodactylus gasterostei were dependent on temperature variability and the host’s immune response. In variable conditions, an established G. gasterostei was better able to adapt to a changing environment than the host’s response causing higher peak infection intensities (Chapter 3 and 4). Temperature, and not photoperiod, was the major cause of circannual rhythm in host immunity (Chapter 6). Co-infection between G. gasterostei and A. foliaceus, revealed higher gyrodactylid infection peaks compared to hosts infected with G. gasterostei alone suggesting that immunomodulation by A. foliaceus. Lastly, pathology, rather than drag, reduced burst and long-term swimming performance of sticklebacks infected with A. foliaceus. Many of the factors highlighted have implications for aquaculture. High aquaculture feeding regimens, resulting in higher body condition, co-infection and temperature, all could severely increase morbidity and mortality of fish in a parasite species dependant manner.