Every year, billions of animals migrate long distances. If animals with parasites are less likely to reach their destinations, then migration may allow populations to escape habitats where parasites build up over time. Understanding how animal migration affects the spread of disease is important to predicting disease risk, including to humans. This study focuses on monarch butterflies as a model system and explores the impacts of parasites on butterfly populations that migrate and populations that have recently lost their migratory habits. This study will also examine how non-migrating populations may influence the risk of having parasites by the migrating forms in areas where they come together. Field and experimental studies will also examine how these microscopic parasites influence how long butterflies live and their flight ability. Through the high visibility and public appeal of monarchs, this project will support the participation of citizen scientists on a continental scale. The investigators will also mentor and train undergraduate and graduate students and will develop web and classroom based educational materials on host-parasite relationships and animal migration. This work will provide additional opportunities for middle school students and support k-12 activities. Finally, this work will inform efforts to conserve monarch butterflies and our understanding of how migration may influence the importance of parasites across many systems. Using a monarch butterflies and protozoan parasite system, the goal of this study is to examine how migratory and sedentary behaviors influence transmission risk and severity of infection. Research activities will integrate (i) an analysis of continent-wide citizen science databases to monitor monarch butterfly abundance and infection; (ii) field and experimental studies to test how monarch migratory behavior depends on non-native milkweeds and infection status, and to quantify parasite transmission rates; (iii) molecular genomics to ask whether genetic changes in sedentary populations might reinforce the loss of migratory behavior; and (iv) mathematical modeling to track how host and parasite populations respond to migratory vs. sedentary strategies. Information on the mixing of non-migrants and migrants, inferred from stable isotope analyses of field-collected monarchs, will inform mathematical models of interacting sedentary and migratory populations, to explore the consequences for disease spread and the persistence of migratory populations. The project will develop theory for how overlapping migration strategies alter, and are themselves affected by, parasite infection, and will shed light on the evolution of migration by characterizing genetic changes that accompany the recent formation of sedentary populations.