Aerosols and droplets have long been recognized as routes of microbial transmission. Measles, influenza, smallpox, and tuberculosis are all known to be transmissible between patients by droplets; measles can also be spread by aerosol. In the laboratory, aerosol promulgation of tularemia, rickettsiae, viral hemorrhagic fevers, and many other agents is a threat to the microbiologist (DHHS, 1999). Artificially generated aerosols of anthrax and other agents are high on the list of terrorist attack options. Decreases in human tuberculosis and virtual elimination of diseases such as measles and smallpox from common medical experience, as well as the development of enhanced methods for protecting laboratory workers (ironically using technology developed during the U.S. biowarfare program), have resulted in a loss of appreciation for this route of infection. Yet the U.S. and Soviet biological weapons programs were based largely on the properties of selected agents for causing large-scale infection of human populations under the proper meteorological conditions and with carefully developed methods of aerosol dissemination. Moreover, the terrorist could attack enclosed environments, such as stadiums or large buildings, to negate the meteorological factors that degrade a small-particle aerosol.
Biological agents have not seen widespread use in warfare, so it is not surprising that there is skepticism as to their efficacy. It is generally not appreciated that the U.S. program in offensive biological warfare (terminated in November 1969) rigorously tested each step in the link between a microorganism selected by several criteria and the delivery of a credible biological attack (U.S. Congress, 1993a, 1993b; Rosebury, 1947; Hersh, 1968; McDermott, 1987; Cole, 1997; Sidell et al., 1997; IOM, 2001c). Tularemia is an excellent example because extensive information on this agent is available in the published literature, records of congressional hearings, and the popular press. From its initial isolation (Francis, 1921) this organism was notorious for causing infections in the laboratory, a frequent hallmark of aerosol infectivity. The agent’s aerosol properties were studied intensively, and methods were found to enhance its stability in storage and in aerosols. Animals and later humans were challenged with graded doses of the bacterium delivered in different particle sizes to establish the quantitative properties of these aerosols. Open-air dissemination was mimicked using a surrogate organism, Serratia marcescens, and this effort confirmed that an organism with the aerosol stability and infectivity of Francisella tularensis could cause mass casualties over large geographic areas, provided attention was given to meteorological conditions. The areas affected could reach thousands of square kilometers. The resulting environmental retransmission from the large numbers of different nonhuman mammalian and arthropod species that would be infected in a tularemia attack cannot be