nature's bioterrorist.

Early last September, an eleven-year-old girl from Kamphaeng Phet, a remote village in Thailand, developed a high fever, a severe cough, and a sore throat. She lived with her aunt and uncle in a one-room wooden house--not much more than a hut on stilts. The family had fifteen chickens, which wandered freely beneath the plank floor, where the young girl often played and slept. Then, at the end of August, the chickens died. Within days, the girl was sick, too. Her aunt took her to the hospital, but the fever kept rising. The girl's mother, who lived near Bangkok, where she worked at a factory, rushed to her bedside; sixteen hours later, her daughter was dead. In keeping with Thai custom, she was cremated immediately.

Avian influenza is nothing new in Thailand, or anywhere else where poultry are raised. Veterinarians often refer to it as the fowl plague, because in one form or another the disease has killed millions of chickens, turkeys, and other birds over the years. In 1983, the virus raced so rapidly through the Pennsylvania poultry population that health officials there were forced to slaughter nearly every chicken in the state. Until recently, however, humans rarely became infected with this type of virus. It had happened fewer than a dozen times since 1959, and in each case the illness was mild. But the strain that killed the girl from Kamphaeng Phet is different; in the past two years, it has caused the deaths of hundreds of millions of animals in nearly a dozen Asian countries. No such virus has ever spread so quickly over such a wide geographical area. Most viruses stick to a single species. This one has already affected a more diverse group than any other type of flu, and it has killed many animals previously thought to be resistant: blue pheasants, black swans, turtledoves, clouded leopards, mice, pigs, domestic cats, and tigers. Early in February, nearly five hundred open-billed storks were found dead in Thailand's largest freshwater swamp, the Boraphet Reservoir. And the disease is no longer limited to Asia. In October, customs officers at the Brussels airport seized two infected eagles that had been smuggled from Thailand and destroyed them, along with the other animals held in quarantine at the airport.

This virus also kills people--so far, forty-two have died, including thirteen Vietnamese since Christmas. Those deaths represent more than three-quarters of all known avian-flu infections--an ominous mortality rate. Strains of influenza are named for two proteins on their surface that latch onto respiratory cells and permit the virus to invade them, and, if this strain, known as H5N1, becomes capable of spreading efficiently among humans, it will kill millions. Public-health officials in Asia, still reeling from the crisis created in 2003 by sars (severe acute respiratory syndrome), have struggled to contain the burgeoning epidemic. Yet the task is immense. sars was caused by a virus that turned out to be less deadly, less contagious, and far less aggressive than the flu. The threat was never as great. There are six billion chickens in Southeast Asia, and millions of households depend upon them, for income as well as for food; preventing this flu virus from spreading has become all but impossible. By early February, during Tet, Vietnam's biggest holiday, officials began posting livestock inspectors on major highways. Last week, officials banned poultry-raising in Ho Chi Minh City.

Vigilance is one of the few weapons available. Two weeks after the girl died in Kamphaeng Phet, Thai epidemiologists were asked to visit a hospital near Bangkok, where a woman had symptoms that matched those caused by the virus. It turned out to be a false alarm, but while the investigators were there a nurse took one of the doctors aside and mentioned that another woman had just died of similar flu symptoms. The death hadn't been reported, but the victim's last name sounded familiar, and so did the name of her village.

"It was just a fluke," Scott Dowell told me not long ago over tea in his office, on the sprawling campus of the Thai Ministry of Public Health, in the suburbs north of Bangkok. "Sure enough, the woman was the mother of that eleven-year-old girl. We would never have known if that nurse didn't happen to mention it." Dowell is the director of the Thailand office of the International Emerging Infections Program, which was established by the Centers for Disease Control in 2001. The group, which was among the first to identify sars when that new virus appeared, is a front-line outpost dedicated to preventing the spread of infectious disease in a world where modern travel, global commerce, and porous borders mean that any new pathogen--whether a lethal strain of influenza, H.I.V., or the synthetic creation of a bioterrorist--may be no more than a plane ride away. Thailand is not nearly as poor as some of its neighbors, but its fertile climate and lengthy borders with Cambodia and Laos--neither of which has a credible public-health system--produce a continual stream of odd, and often deadly, infectious diseases. The Nipah virus, which is named for the village in Malaysia where it was discovered, in 1999, has been endemic; so have dengue and leptospirosis, a bacterial infection that affects humans and animals. Thailand also has serious epidemics of tuberculosis and H.I.V.

Now there is little on people's minds but the flu. "Our Thai colleagues called us and said, 'Hey, do you want to go to Kamphaeng Phet?' " Dowell said. "And on the way up there they told me the story. And I just said, 'We absolutely have to get hold of this virus.' This could be the first clear case of person-to-person transmission, the beginning of something very significant, something terrifying. So the girl had died and been cremated, and the mother was dead and embalmed. She was literally at the wat, awaiting cremation. The Thai doctor who was riding in the car with me was calling on his mobile phone to the team there, telling them to do whatever they had to do to hold that embalmed body out and take some blood or a tissue sample."

A pandemic is the viral equivalent of a perfect storm. There are three essential conditions, which rarely converge, and they are impossible to predict. But the requirements are clear. A new flu virus must emerge from the animal reservoirs that have always produced and harbored such viruses--one that has never infected human beings and therefore one to which no person would have antibodies. Second, the virus has to actually make humans sick. (Most don't.) Finally, it must be able to spread efficiently--through coughing, sneezing, or a handshake. For H5N1, the first two conditions have been met; it's new and it's deadly. On the ride to Kamphaeng Phet, Dowell couldn't stop wondering whether the virus had met the third condition, too.