bioterrorism, are equally important. So is the ability to understand the limits of the bioterrorism forensics process at any given moment and to accurately interpret and communicate results.

An Approach to Defining Bioterrorist Threats

Pathogenic microorganisms and the toxins produced by living organisms pose a threat to national security whether they occur in their natural state or are released in bioterrorism attacks. In either case, the greatest threats to human health in the United States come from emerging and reemerging infectious agents that sporadically occur in nature. The population is highly susceptible to such infectious agents, and the mortality rates among infected individuals can be high. Such agents in a bioterrorism attack could easily be spread to large numbers of individuals (Peters, 2002).

As part of a risk analysis, one can classify infectious agents and diseases in relation to these sorts of factors. Thus an eradicated disease agent to which there is currently a high degree of susceptibility, for which there is a high rate of mortality among infected individuals, that can be spread as an aerosol, and that can continue to be spread via contagion—in effect, a worst-case disease—could inflict the most casualties. Smallpox is such a disease, and it is at the top of the list of biological agents that may pose a threat. Once measles is eliminated (Hilleman, 2001) it will join smallpox in this category if immunization against measles is halted (as was done for smallpox) and the population becomes highly susceptible. This has important policy implications for the continuation of immunization against a disease agent after elimination of its natural occurrences.

Previously circulating pandemic influenza strains, most notably the 1918 Spanish influenza (Taubenberger, 2000) and the 1957 Asian influenza (Cox and Subbarao, 2000), and influenza strains of novel subtypes—e.g., the 1997 H5N1 strains from Hong Kong—have pandemic potential in humans. Ebola and hemorrhagic fevers (the causative viruses of which, however, are less easily spread from person to person than influenza viruses) would also have the characteristics of rare diseases that are communicable, to which there is a high degree of susceptibility, and for which there is a high rate of mortality among infected individuals. A genetically engineered pathogen could also have these characteristics and would need to be viewed as being among the most serious potential biological threats. The difficulty is that such genetically engineered pathogens could be created from virtually any biological pathogen or even vaccine strain; thus it will be challenging to develop vaccines or therapeutic antimicrobial agents in advance of a bioterrorism attack.

Because eradiated or genetically engineered agents often do not occur naturally or are difficult to obtain from nature, the best source for terrorists is a research facility. It is thus appropriate to impose significant restrictions in terms of oversight and apply stringent security precautions for biological agents that

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