Climate warming has far-reaching implications for host-parasite interactions, from direct effects of temperature on host and parasite physiology to temperature-mediated indirect effects on interacting species. The capacity of migratory animals to track warming climates is critical to their persistence and shapes their overlap with parasites, but studies have rarely considered the combined effects of temperature on physiology and phenology in a multi-species context. Given the central role of some migrants in providing ecosystem services, or as dispersers of zoonotic pathogens, an integrated understanding of warming effects on migratory host-pathogen dynamics has crucial conservation and public health implications. In this dissertation, I use experiments and mathematical modeling to investigate how climate warming impacts infectious disease outcomes in migratory hosts. My empirical work focuses on the interaction of monarch butterflies (Danaus plexippus), an iconic migratory insect of conservation concern, and their protozoan parasite (Ophryocystis elektroscirrha). First, I quantified temperature effects on monarch immunity, fitness, and infection using constant-temperature growth chamber experiments. At the hottest temperature treatment (34°C), monarch size and survival declined, and strikingly, the parasite failed to infect, suggesting that extreme heat decreases both host and parasite performance. Next, I assessed how warming and host plant chemistry alter monarch fitness and infection outcomes. I reared monarchs in ambient-temperature and temperature-elevated field plots, using two milkweed (Asclepias) species with different concentrations of defensive compounds that mitigate costs-of-infection in monarchs. Elevated temperatures increased the costs of infection for monarchs, suggesting that moderate warming erodes the protective effects of milkweeds against parasitism. Finally, I developed a mathematical model of a zoonotic vector-borne disease (West Nile Virus) in a migratory bird to predict how climate warming will impact infection outcomes. Failure of migratory hosts to update their arrival phenology exacerbated warming-driven increases in transmission, and lower mosquito survival under severe warming advanced the peak timing of transmission, with implications for human exposure risk. Collectively, this work sheds light on the multiple mechanisms through which warming could influence migratory-host pathogen dynamics, highlighting the need to account for indirect effects of warming on interacting species to anticipate and mitigate the negative impacts of infectious diseases.