Behavior is the first line of defense against parasites and pathogens. Behavioral defenses allow organisms to minimize contact with infectious agents, and the concept of the “behavioral immune system” was developed to encompass a range of evolved behaviors that help minimize the fitness costs of infection. The COVID-19 pandemic has made the term “social distancing” a household term; however, distancing and many other avoidance strategies are employed by a wide range of organisms to combat infectious agents. In this seminar, we examine our current understanding of the behavioral immune system across the diversity of animals, including humans. Specifically, we explore: (1) the mechanisms of behavioral immunity; (2) the ecological, evolutionary, and epidemiological consequences of these behaviors; and (3) key costs of behavioral immunity that maintain intra- and interspecific variation. To do this, we discuss and synthesize the scientific literature on the behavioral immune system, drawing parallels to work on the physiological immune system. The first weeks of the course focus on instructor-selected papers, and subsequent weeks incorporate student-selected papers.