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Biological-warfare agents are, in my judgment, the biggest menace we currently face, but not all such agents are created equal. Bioweapons perfected by the major powers in the immediate postwar period included diseases of plants, animals and humans. They were further divided into diseases such as anthrax that are merely infectious-caused only through direct exposure to a weaponized bacterium or virus-and those like smallpox that are also contagious, or spread from one person to another.

The five deaths and six inhalational anthrax illnesses caused by anthrax-bearing letters-probably from a single domestic terrorist incited by the events of September 11-have taught us a lot about inhalation anthrax that we did not know before. Quite apparent, and not surprising, is the fact that the spores may reside for weeks and months in the lung before vegetating-even in the presence of effective antibiotics-and if antibiotics are withdrawn, an ensuing infection is usually lethal within days unless promptly treated. The likelihood of prompt treatment has, of course, increased greatly, since a disease almost unknown to the ordinary medical practitioner is now at the top of everybody’s alert list. Yet anthrax remains a concern because in the form of spores it is so durable. The dissemination of dry spores has been achieved in Russia and the United States and perhaps elsewhere, and tons of anthrax spores have been produced and weaponized-for instance, in 1991 in Iraq.

But my chief concern is not a handful of anthrax letters, or even a hypothetical mass mailing of 10,000 anthrax letters. What worries me most are biological agents that are contagious as well as infectious.

Take smallpox, for example, a viral disease that spreads rapidly and kills 30 percent or more of the people it infects. Through foresight and aggressive action on the part of the World Health Organization, smallpox was deemed eradicated in 1980. Two stocks of smallpox were officially maintained-one in the United States and one in what is now Russia-but testimony from one of the workers in the Russian biowarfare program attests that the Soviet Union had secretly weaponized smallpox. The Soviets apparently had numerous ballistic-missile warheads filled with biowarfare agents; some of those agents may have been stolen or diverted. Since the United States has never had access to Russia’s former military biowarfare installations, we just don’t know the full extent of the program and the degree to which agents remained under the government’s control.

It is also likely that some individual researchers in the United States and elsewhere, whether in military or civil programs, did not destroy their stocks of smallpox virus when their nations signed the Biological Warfare Convention of 1972 but kept some for a rainy day-perhaps without any malevolent intent. Some of these stocks may have fallen into the hands of terrorist groups; stored, they could be multiplied by the same techniques used to grow viruses for human or animal vaccines and could be available for widespread dispersion from moving cars or trucks. Depending upon the planning and organization for countering such an attack, the initial infection of 100,000 people might lead to infection of many tens of millions and, given the smallpox fatality rate, the death of 30 million people within four months.

Against smallpox, as with most other viruses, antibiotics are useless; there is no effective treatment after symptoms appear. There is, however, an effective vaccine. But though it was long mandatory in the United States, smallpox vaccination was abandoned here in 1972. At the time I argued strongly in the President’s Science Advisory Committee that the country would be too vulnerable to intentional attack, and that vaccination should be continued, despite the two or three people per year who might die from vaccine side effects. The government didn’t heed this urging, but it preserved in liquid nitrogen a stock of about 15 million doses of the smallpox vaccine.

By conventional wisdom, this stockpile is not enough to vaccinate the entire U.S. population. But recent experiments have shown that the vaccine is effective in doses five times as dilute as normal, and a further economy may be achieved by adjustment to the way the vaccine is delivered. What’s more, in late March 2002, vaccine maker Aventis Pasteur announced that it had some 85 million additional doses of smallpox vaccine in storage and agreed to donate them to the U.S. government; these also can be extended by dilution. In other words, there is now more than enough for every U.S. resident (though the government has not, so far, reinstated widespread vaccination), even without the new smallpox vaccine currently in development.

In the case of smallpox we got lucky. But there are many potential biowarfare agents, such as Burkholderia mallei, a contagious bacterium that causes a deadly disease called glanders, for which there is no vaccine. They might be disseminated within large buildings-and distributed by the circulating air in heating, ventilating and air-conditioning systems-or outdoors, to expose whole cities. And in the case of an outside release, even people who were indoors with the windows shut would be at risk of exposure, as air tends to leak through tiny gaps and cracks in most buildings.

Simply replacing the normal air filters used in most ventilation systems with high-efficiency filters-which are already widely available-could offer some degree of protection against a number of bioweapons. Such filters aren’t a perfect solution, though, because contaminated air still circulates for a time before getting routed through the filter. But if living and working spaces were maintained at positive pressure so that any leaked air flowed out instead of in, high-efficiency filtration of “makeup air”-that required to maintain the positive pressure-could reduce the risk of bioweapon exposure by a factor of a thousand or more.

I strongly advocate such positive-pressure protection. It could be applied not only in buildings but in public transport or even private automobiles as well. And implementing it is relatively cheap. At a typical annual office rental rate of about $9,000 per employee, it amounts to an added cost of about $9 per employee each year-a burden many judge is overwhelmed by its routine benefits in reducing allergies and normal transmission of communicable diseases.

Some argue against filtration and positive-pressure protection because they offer imperfect solutions-they don’t protect people who are outside, and they don’t protect against toxic chemicals. The demand for perfection often stands in the way of tremendous benefit.

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