Many natural systems are being affected by increasing environmental temperatures. Evolutionary potential (genetic variability) and plasticity (genotype x environment interaction; GxE) of populations will ultimately determine their survival in changing environments. While plasticity can enable organisms to respond quickly and effectively to such change, adaptive evolution, i.e. evolution of reaction norms (GxE) is necessary in the long-term to cope with altered environmental conditions. Assessing the potential of thermal reaction norms to evolve is very relevant to understand how species can deal with rapid climate change. In addition, it is largely unknown how environmental change interacts with selection imposed by parasites. Here, we use a fish host-trematode parasite model system from marine coastal ecosystems to investigate evolutionary potential and plasticity of thermal reaction norms of key fitness traits, and the temperature-dependence of genetic correlations and tradeoffs among life history, condition and parasite resistance traits. Using a quantitative genetics approach, we extend the GxE interactions of single species to multiple species (GxGxE), from which we can predict infection outcomes and evolutionary dynamics depending on the genetic variability of host and parasite populations.