In the wake of the Sept. 11 attacks on New York City and the Pentagon, Americans find themselves looking over their shoulders at every turn. Is it safe to fly? Safe to gather in a public place? Safe even to drink our own tap water?

Unfortunately, our escalating worries may not be entirely unwarranted. On Monday, Secretary of Defense Donald H. Rumsfeld warned of the possibility of a biological attack on the United States. Society’s standard defense against such attacks—that no one would be so inhuman as to carry them out—holds less currency now that we have seen terrorists make deadly weapons of passenger planes. Experts believe that the use of biological weapons could potentially present an even greater threat to human existence than nuclear weapons did a few decades ago.

Crude biological agents have been used in war for centuries. But today’s fears are of the deliberate creation of a virulent plague. Iraq, Iran, Russia, North Korea, Libya, Syria and Sudan are all known to have biological weapons programs. The U.S. and the Russia have both worked on the development of anthrax, a bacterium that is spread among livestock and that poses significant dangers to humans. Terrorists could even reverse one of humanity’s greatest achievements by reintroducing smallpox, which has been eradicated by a sustained global health effort and which is no longer treated as a danger requiring immunization.

Unfortunately, America may not be adequately prepared against a biological attack. This June, American military experts gathered for a mock germ-warfare exercise termed “Dark Winter.” The simulation began with a single case of smallpox virus in Oklahoma. Despite the best efforts of the trained personnel involved, 16,000 Americans were infected in the simulation and 6,000 ultimately died.

How can we best protect Americans from this new threat? No defense can ever be foolproof, and a free society will always be vulnerable to those who would abuse its liberties. As a result, preparation appears to be the best means of crisis prevention. The U.S. must gather as much information as possible about the biological weapon stores of other nations, train infection control teams and secure stores of vaccines and poison antidotes. Simulated scenarios like “Dark Winter,” rehearsed on a broader scale, can help the U.S. improve its response to a possible attack. An efficient and effective public health care infrastructure will also be crucial. Physicians and emergency care workers should be trained to recognize known biological agents, and good communications will be needed to allow for the tracking of a new disease. An early-warning system could spell the difference between a small outbreak and major epidemic.

At the same time, however, we must be careful not to allow doomsday prophecies and other scare-mongering to provoke extreme reactions. Stockpiling huge quantities of vaccines or antibiotics may be expensive and ultimately ineffective, especially in an age when mutant germs and antibiotic-resistant microbes can be easily developed. Instead, investing in intelligence missions, research and the public health infrastructure should provide more value and security in the long run.

The greatest destructive force of biological weapons depends on the panic and ensuing chaos caused by their initial release. Rational, careful planning can ensure that any biological attack is contained before it has the chance to replicate its terror.