Public health experts worry that the bird flu outbreak in Asia, Russia, and Kazakhstan could spread to humans and cause a global pandemic that kills millions. But avian influenza is just one of several lethal infections challenging world health. The emergence of many new diseases in recent years and the re-emergence of old ones are shattering the illusions of medical researchers about their ability to outwit them.

The news may be full of war, murder, and other mayhem, but infectious diseases are still the world's leading killer. The World Health Organization says scourges like AIDS, SARS, tuberculosis, cholera, and countless others cause one of every four deaths. The physician who directs U.S. government infectious disease research, Anthony Fauci, has a favorite saying: "In many respects, nature is the worst bioterrorist."

Although developing nations bear the brunt, the official in charge of U.S. emergency public health preparations, William Raub, says no one is safe in the modern world because germs spread rapidly. "With the increased means of transportation, with the increased access that people have to it, it is really enhancing this as a public health problem," he explained.

Medical experts fear it is only a matter of time before the bird flu virus mutates to a strain fatal to people and becomes the next outbreak to leap borders quickly. But even if it does not, recent history shows that some other infection is always looming.

William Raub recalled the SARS epidemic of 2003. "We found ourselves with a disease we had not seen before," he noted. "We therefore had no diagnostics, no drugs, and no vaccines. But we had it rapidly spreading around the world because of the ease of international travel."

New organisms are not the only threats, because of the biological reality that old germs like the tuberculosis bacterium can mutate to tolerate established drugs and regain virulence.

Medical masks help prevent the spread of disease

The economic burden of infectious disease is enormous, as SARS proved to China, Hong Kong, and Toronto in lost tourist spending. But not only tourism is at stake, for diseases like AIDS strike people down in their most productive years.

"Increasingly, studies are showing that good health can contribute to economic growth and bad health can constrain it," says George Fidas, co-author of a recent U.S. government intelligence report on communicable illnesses. "Health also is a global and national security issue because it can contribute to insecurity at the personal, the communal, the national, and even the international levels."

But mankind is not helpless against microbes. Global cooperation eradicated smallpox in the 1970s and slow progress is being made toward eliminating polio, both thanks to vaccines. In addition, the United States has massively boosted its public health spending because of the spread of anthrax powder by U.S. mail in 2001 soon after the terrorist attacks on New York and Washington.

The SARS outbreak two years ago was a test for America's improved detection capabilities and its coordination with international public health agencies. In just months, the SARS organism was identified, the first step toward drug and vaccine development, and containment measures put in place.

But in a world of tight budgets, there are limits to response. Anthony Fauci, the head of the U.S. government's infectious disease research, points out that the avian flu could push those limits. "We've accomplished a lot, but we still have a considerable way to go," he says. "You can never be completely prepared."

Dr. Fauci's agency has reported promising results with an experimental avian flu vaccine, but the problem will be getting enough doses produced, a frequent obstacle for infectious disease pharmaceuticals. Dr. Fauci says drug companies need motivation to develop them because the markets are unproven. "It is not a great financial incentive for drug companies to get involved in making vaccines and drugs for microbes that we may not ever have to use these vaccines or drugs for," he explains.

To remedy this, a U.S. law took effect last year to provide such incentives to drug companies and Congress is considering a second one to strengthen them. Together, they promise that the government will buy the pharmaceuticals and provide patent and liability protection. "Approximately 90 percent of the world's medicines are created in the United States," says Charles Ludlum, the recently retired legislative aide who authored both measures. "Basically if we don't set up a system of incentives, then we will not be able to get the products. We will basically be unprepared."

Whether or not the world is prepared, most medical experts believe a major epidemic for which no treatment exists will strike at some point.