Monarch caterpillars are specialist feeders on milkweeds, from which they sequester toxic cardenolides, protecting them against predators. Their bright colours advertise toxicity. North American monarchs make an annual migration; millions of them travel from the USA and Canada to overwinter in central Mexico. They are an ideal system to study the effects of multitrophic interactions and migration behaviour on the ecology and evolution of infectious disease. Monarchs are commonly infected with a protozoan parasite. Our studies have shown that milkweed chemicals reduce parasite growth, transmission, and virulence and are used by monarchs to reduce infection in their offspring, although this may also select for more virulent parasites. Studies have also shown that seasonal migration is an important determinant of parasite prevalence through migratory culling, when the most heavily infected individuals are weeded out during the autumn migration, and migratory escape, when they escape contaminated environments, reducing infection probability. Conservation efforts have increased the planting of non-native medicinal milkweeds in North America, and monarchs have increasingly formed sedentary populations, reducing migration rates. This has increased parasite prevalence in non-migratory populations. Integrating studies on multitrophic interactions and migration behaviour are necessary to determine their long-term effects on parasite dynamics and host and parasite evolution in monarchs.