This essay concerns the current intersection of national security and public health in the United States. It argues that over the course of the past three decades, a new way of thinking about and acting on the threat of infectious disease has coalesced: for public health and national security officials, the problem of infectious disease is no longer only one of prevention, but also—and perhaps even more—one of preparedness. The essay describes the process through which a norm of preparedness came to structure thought about threats to public health, and how a certain set of responses to these threats became possible. The story is a complex one, involving the migration of techniques initially developed in the military and civil defense to other areas of governmental intervention. The analysis is centered not on widespread public discussion of biological threats but, rather, on particular sites of expertise where a novel way of understanding and intervening in threats was developed and deployed. It focuses in particular on one technique, the scenario-based exercise, arguing that this technique served two important functions: first, to generate an affect of urgency in the absence of the event itself; and second, to generate knowledge about vulnerabilities in response capability that could then guide intervention. More broadly, the scenario-based exercise is exemplary of the rationality underlying the contemporary articulation of national security and public health.