Potential biologic weapons share several characteristics. Ease of acquisition and production is a primary consideration. Other ideal properties include the potential to be aerosolized (particle sizes of 1 to 10 μm) and dispersed over a wide geographic area, as well as resistance to sunlight, dessication, and heat. The potential to cause lethal or debilitating disease and person-to-person transmission are important features, as is lack of effective therapy or prophylaxis.

Any small or large outbreak of disease merits evaluation as a potential biologic event. Unusually high rates of disease, as well as unusual clinical syndromes (e.g., a cluster of life-threatening pneumonia in otherwise healthy adults), should signal a warning. Once the case definition and attack rate have been determined, an epidemic curve can be calculated based on the number of cases over the course of time. The epidemic curve in a biologic event triggered by a point-source exposure most likely would be compressed, with a peak reached in a matter of hours or days. The occurrence of a second curve peak is possible with contagious agents, as a result of person-to-person transmission. The steep epidemic curve expected in a bioterrorism attack is similar to that seen with other point-source exposures, such as foodborne outbreaks.

Several epidemiologic clues may be helpful in determining whether further investigation into an outbreak as a potential biologic attack is warranted. A large epidemic, especially one occurring in a discrete population; more severe disease than expected for a given pathogen; and a disease unusual for a given geographic area (e.g., pulmonic tularemia in an urban setting) represent major indicators. Multiple simultaneous epidemics of different diseases, outbreaks with both human and zoonotic consequences, and unusual strains or susceptibility profiles are additional helpful parameters. Variable attack rates as a function of an agent released relative to the interior or exterior of a building also are useful. Although most bioterrorism attacks will be covert, intelligence revealing plans for an attack, terrorist claims of a deliberate release, or direct physical evidence of an attack obviously point to such an event.

The emergence of mosquito-borne West Nile (WN) virus encephalitis in New York City in the summer of 1999 is an example of a naturally occurring outbreak that had elements of a potential bioterrorist attack. This outbreak represented a disease occurring in an unusual (previously nonendemic) area as well as one with zoonotic (birds) in addition to human consequences. It marked the first documented appearance of WN virus in the Western hemisphere and the first arboviral outbreak in New York City since the yellow fever epidemics of the nineteenth century. A large avian die-off, affecting primarily crows, preceded the outbreak in humans by at least several weeks.

In addition to anthrax, critical biologic agents include plague, tularemia, smallpox, viral hemorrhagic fever, and botulinum toxin (Table 138.1). These credible biologic agents are discussed further.