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Could terrorists, intent on causing as much harm and societal disruption as possible, use new biotechnology processes to engineer a virulent pathogen that, when unleashed, would result in massive numbers of dead? Mark Williams, in his article “The Knowledge,” suggests we should be contemplating this doomsday scenario in the 21st century. Williams’s article might make you sleep less soundly, but are the threats real? The truth is that we do not really know.

Part of the problem is that even if terrorists could create new pathogens virulent to humans, it’s not at all clear that they could “weaponize” them – that is, put the pathogens into a form that is highly infectious to humans and then disperse them in ways that expose large numbers of people.

Past experience suggests that this is not an easy task. During World War II, the Japanese dropped plague-infected materials on Chinese cities, to limited effect. In 1979, the Soviets caused 66 deaths from anthrax by accidentally releasing it from a bioweapons facility in Sverdlovsk. In 1984, the Rajneeshees cult contaminated salad bars in the Dalles, OR, with salmonella, but their actions killed no one. In 1993, the Aum Shinrikyo cult failed to kill anyone after carrying out multiple attacks with anthrax in Japan. Finally, the 2001 anthrax letter attacks in the U.S. killed five people. These were all frightening events. They were not, however, grave threats to national security.

Yet estimates of bioweapons dangers tend to be dire, like those in Williams’s article. The truth is that the data are too thin to make accurate projections of the effects of bioweapons attacks. I surveyed seven separate estimates of fatalities from a projected anthrax attack. The lowest estimate, by Milton Leitenberg, ranged from zero to 1,440 dead per kilogram of anthrax used, while the highest, by Lawrence Wein and others, put fatalities between 123,400 and 660,000 per kilogram of anthrax. Most of these estimates were made on the basis of little actual data.

To predict accurately the effects of bioweapons, data are needed on the amount of agent required to infect a person, the percentage of people who survive an infection (which depends on the health of the population), the transmission rate if the agent is contagious, the ability to aerosolize and disperse an agent effectively (which depends, in turn, on climatic conditions), the environmental stability of an agent, the population density, and the abilities of the public-health system, including when an attack is detected and whether prophylactics, vaccines, or antidotes exist and, if so, in what quantities.

For any one pathogen – even one familiar to us, like smallpox and anthrax – not all of these variables are known, and therefore quantitative predictions are not possible with a high degree of certainty. In the words of the U.S. National Academy of Sciences in a 2002 report, “these factors produce an irreducible uncertainty of several orders of magnitude in the number of people who will be infected in an open-air release.”

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