Focusing on British Columbia during the mid-twentieth century, this article illuminates how North American medical, public-health, and law-enforcement professionals used the "reservoir" metaphor in efforts to control venereal disease (VD). It traces the transition from a pre-Second-World-War paradigm of VD eradication - what I call an epidemio-logic - focused on the single reservoir of female sex workers, to one concerned with several groups, including the White "male homosexual." The article also demonstrates how conceptualizing VD control in terms of human reservoirs led to analogical reasoning, improvements and setbacks to disease-control efforts, shifting understandings of infection risks, and changes to the built urban environment.