Monarch butterflies inhabit multiple islands and continents worldwide and are best known for undertaking a spectacular migration in parts of North America. Monarchs are affected by a neogregarine protozoan parasite throughout their range and prevalence varies inversely with host migratory distances, with highest prevalence in populations that breed year round and do not migrate. Mechanisms that could generate lower infection rates in migratory populations include environmental differences between sites and effects of migration on host resistance and survival. Here we summarize results from small-scale experiments and field data that show that parasites pose significant costs for host migration success, and that multiple factors affect host susceptibility to infection in migratory populations. Furthermore, experimental results suggest that warmer temperatures such as experienced by monarchs that breed year-round in tropical locations can lead to lower average spore loads and greater relative longevity of infected hosts. Collectively, these results have relevance for understanding how seasonal changes linked with migration can affect host susceptibility and parasite transmission, leading to variation in infection rates within and among populations. Because many animals migrate seasonally long distances, these results are also relevant to understanding how climate warming and other human activities that alter host migratory patterns can affect pathogen spread.