More than two years before September 11 and the anthrax attacks that followed, the CDC had been designated by Congress to prepare for the likelihood of a bioterrorism attack. Among numerous other tasks related to this directive, the CDC examined a myriad of bacteria and viruses that are pathogenic to humans and determined which of these might be used as biological weapons. A panel was assembled with experts in public health, infectious disease, military bioweaponry, members of the intelligence communities, and law enforcement to determine the agents that were viable threats as biological weapons (Table 11–1). Four criteria were considered in selecting these agents. As stated by the CDC, the four points were:

• • Public health impact using two measures: morbidity and mortality
  
• • Delivery potential to large populations based on two primary considerations: ability to mass produce and distribute a virulent agent and potential for person-to-person transmission of the agent, and the stability of the agent was considered
  
• • Public perception as related to fear and potential civil disruption
  
• • Special public health preparedness needs based on stockpile requirements, enhanced surveillance, or diagnostic needs

Based on these considerations, the working group developed a list of seventeen pathogens or types of pathogens that were felt to pose legitimate threats. These pathogens were subdivided subsequently into Category A, B, or C agents according to the degree and nature of the threat posed (Table 11–2).