Over the past year, historians of medicine have found our discipline invested with a new sense of relevance. In trying to make sense of epidemics past and present, many of us have been substantially influenced by Charles Rosenberg's 1989 Daedalus essay, "What Is an Epidemic? AIDS in Historical Perspective." Writing in the middle of another unfolding global pandemic, Rosenberg suggested that all epidemics possessed similar forms of social choreography, and that applying a narrative framework could help to understand their sequence, structure, and social impact. This issue of the Bulletin offers contributions from thirteen scholars working in various geographic, chronological, and thematic areas that engage with Rosenberg's fundamental historical question about what defines an epidemic, although the question takes on different forms, and different forms of urgency, in each of their works.