In the aftermath of the Spanish Flu pandemic of 1918-19 epidemiologists began appealing to “herd immunity” as an explanation for the rise and fall of epidemics. Originally invoked to explain experiments with mouse populations exposed to disease, by the late 1920s herd immunity was being spoken of as an observable quality of human communities in response to diseases such as dipthheria, scarlet fever and influenza. As the century proceeded and a growing array of vaccines enabled the ability to cultivate immunity against once everyday infections, however, herd immunity was increasingly equated by many with a strategy or outcome of mass vaccination. I argue that this long-standing tension between observing and cultivating immunity offers important insights into the controversy surrounding the relevance of the concept during the recent pandemic.