Someplace in Africa, a bug bites a bird.
And sets in motion, months later and a continent away, a chain of events that includes emergency spraying for mosquitoes in the asphalt-and-concrete canyons of New York City; deployment of health officials to collect and test dead birds in several states; a run on bug spray up and down the East Coast; and all-nighters in virology labs from Ames, Iowa, to Irvine, Calif.
The outbreak of illness in New York since August, originally believed to be St. Louis encephalitis, was identified last week as closely resembling two even more exotic diseases, also transmitted by mosquitoes: West Nile virus, usually found in Africa, and Kunjin virus, from Australia. By late last week, 17 confirmed and 20 suspected human cases of the virus, including five deaths, had been reported from New York and its suburbs.
The good news is that in healthy people, the infection usually is no worse than a bad case of the flu, causing fever, headache and no lasting effects. The deaths were among elderly patients.
The bad news is that neither West Nile fever nor Kunjin had ever been found in North America. So scientists are scrambling to understand when it got here, how it got here and what it means.
Did this virus hitch a ride on a bird that crossed the Atlantic by airplane, either deliberately imported or locked in by accident with the luggage? Could a migrating African bird have been blown way off course by a hurricane? Or was the carrier a human, bitten overseas by an infected mosquito?
Whatever the answers, the West Nile-Kunjin outbreak is the latest evidence that the global battle against deadly infections, once believed on the brink of victory, is not likely to end. The complex dance of microbes, animals, human beings and climate is likely to last as long as life survives on Earth, experts say. And the growth of international trade and travel, resistance to antibiotics and global warming are making the disease threat worse.
"What this shows is that many diseases we think of as being not our problem in the United States are in fact our problem," says Dr. W. Ian Lipkin, director of the Emerging Diseases Laboratory at the University of California at Irvine, where the virus was identified in brain tissue from three of the New York victims.
"We, the human species, are in a dynamic tension with microbes," says Dr. Anthony S. Fauci, director of the National Institute of Allergy and Infectious Diseases since 1984. "The question often asked is, 'Which one will prevail?' The answer is, neither will prevail, and both will survive."
Emerging diseases are a hot topic. Fauci, who trained in the 1960s when hubris about the conquest of infection was greatest, recalls the different atmosphere then.
In 1968, as Fauci traveled from New York City to Bethesda to begin a fellowship in infectious disease, he heard news that suggested his career was ending before it had begun: The U.S. surgeon general had declared to Congress that the era of infectious disease was over. Now, he said, medicine should focus on chronic killers such as cancer and heart disease.
"Here I was, heading off to start a career in infectious disease," Fauci recalls. "I felt like a guy going to Miami to become a ski instructor."
Fauci didn't have to worry. The arrival of the AIDS epidemic in the early 1980s, caused by a virus that had passed over from monkeys, was a shocking reminder that infection still could cut a deadly swath through even the most advanced countries.
In 1992, scientists at the Institute of Medicine sounded the alarm about emerging diseases. They reported that deaths worldwide from infectious diseases -- without counting deaths from acquired immune deficiency syndrome -- rose 22 percent from 1980 to 1992.
Since then, there have been splashy stories about worrisome outbreaks of such diseases as Ebola and avian influenza, and best-selling books speculating about the next plague. If natural epidemics aren't trouble enough, there is growing fear that terrorists could mount a biological attack using anthrax, smallpox, botulism or some other pathogen.
It is against this background that the West Nile-Kunjin outbreak is unfolding. (The federal Centers for Disease Control and Prevention calls it a "West Nile-like" virus; Lipkin, who says his lab has mapped more of the genetic material than the CDC has, says it is closer to Kunjin virus.)
The uproar has been touched off by a fuzzy ball about one-2,500th the width of a human hair. Viruses are tiny forms of semilife that invade cells and turn them into virus factories, turning out thousands of copies of themselves. Sometimes they exist at peace with their host, which can become a chronic carrier; sometimes they slaughter cells and cause severe illness.
"That's what fascinates us," says Carol D. Blair, a Colorado State University virologist. "They're so small and so simple. But they get inside your cells and really wreak havoc."
By cycling among mosquitoes or ticks, birds or rodents, deer or humans, viruses ensure their own survival for thousands of years, Blair says.
The latest microbe mystery began Aug. 23 when a doctor at a hospital in Queens, N.Y., reported two cases of encephalitis, or infection of the brain, to the New York City Department of Health. Soon, health investigators found more cases and tentatively identified them as St. Louis encephalitis, a mosquito-borne illness. New York sprayed pesticides to control mosquitoes, which were proliferating after heavy rains.
Wildlife officials had also noticed unusual numbers of birds dying, particularly crows.
Brain tissue from birds that died at the Bronx Zoo was shipped to the U.S. Department of Agriculture's National Veterinary Services Laboratories in Ames, Iowa. Scientists there grew the virus in chicken embryos. They tested without success for a range of unusual ailments such as highly pathogenic avian influenza and exotic Newcastle disease, says Beverly J. Schmitt, head of virology.
They sent the isolated virus to the CDC's laboratory in Fort Collins, Colo., which soon found that it resembled West Nile. Meanwhile, Lipkin's lab at the University of California had tested the human brain samples and come to a similar conclusion. The news started a flurry of scientific exchanges last week on Promed, an Internet group that discusses emerging diseases and keeps researchers from Moscow to Islamabad up to date on the latest pathogenic suspects.
Sometimes the discussion of fevers grows a little feverish. A message posted on Promed, mentioning unusual deaths of "cows," seemed to link the current infection mystery to another, even bigger one, mad cow disease, which sparked trade disputes and led to the slaughter of entire herds.
The message set off another flurry of speculation until a sheepish scientist corrected a typographical error. He had meant to write about unusual deaths of crows.