Parasites commonly interact both with other parasites, and with commensals and mutualists of the host. While interactions among parasites are increasingly studied in disease ecology, we have a more limited understanding of interactions of parasites with mutualists, including those that are defensive symbionts—they increase the performance of their host in the presence of natural enemies including parasites. Defensive symbionts can dramatically impact the survivorship, growth, and reproduction of parasites infecting the same host individual. Yet, the linkage between such interactions within host individuals and parasite transmission across the host population remains an important frontier in disease ecology.