The Veterans Health Administration (VHA) uses the Electronic Surveillance System for the Early Notification of Communitybased Epidemics (ESSENCE) to detect disease outbreaks and other health-related events earlier than other forms of surveillance (1). Although Veterans may use any VHA facility in the world, the strongest predictor of which healthcare facility is accessed is geographic proximity to the patient's residence. A number of outbreaks have occurred in the Veteran population when geographically separate groups convened in a single location for professional or social events. One classic example was the initial Legionnaire's disease outbreak, identified among participants at the Legionnaire's convention in Philadelphia in the late 1970s (2) . Numerous events involving travel by large Veteran (and employee) populations are scheduled each year.