This article explores a tension at the core of the concept of herd immunity that has been overlooked in public and scientific discussions-namely: how can immunity, a phenomenon of individual biological defenses, be made relevant to populations? How can collectives be considered "immune"? Over the course of more than a century of use of the term, scientists have developed many different understandings of the concept in response to this inherent tension. Originating among veterinary scientists in the United States in the late 19th century, the concept was adopted by British scientists researching human infectious disease by the early 1920s. It soon became a staple concept for epidemiologists interested in disease ecology, helping to articulate the population dynamics of diseases such as diphtheria and influenza. Finally, though more traditional understandings of the concept remained in scientific use, in the era after World War II, it increasingly came to signal the objective and outcome of mass vaccination. Recognizing the complexity of scientific efforts to resolve the paradox of herd immunity may help us consider the best distribution of immunity against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2).