GUERRILLA WARFARE

For two months in 1993, otherwise healthy young people had been showing up at hospital emergency rooms on the Navajo reservation in northwest New Mexico. At first they complained of flulike symptoms. Then, within hours, most of them died of acute respiratory failure, literally drowning in their own bodily fluids. Frustrated officials with the Indian Health Service, the University of New Mexico and the state's health department put in a call to the Centers for Disease Control and Prevention in Atlanta. In a matter of days, highly trained specialists--essentially a public-health swat team from the premier U.S. government lab's Epidemic Intelligence Service--were on the scene. Within two weeks, their meticulous detective work had identified the deadly microbe: a lethal variant of a germ known as hantavirus, carried by rodents but never known to be deadly in humans. Armed with that knowledge, health officials launched a multistate public-awareness campaign, and before long the number of deaths plummeted--from 20 in the first few months to just 23 since 1994.

Fast forward to February 1996, when officials at the World Health Organization's Geneva headquarters got word of a possible Ebola-virus outbreak in the remote village of Mayibout, Gabon. It is hard to imagine a more frightening report: ever since the first known outbreak in 1976, the virulent Ebola virus has been near the top of every Central African's list of the worst ways to die. With the 1994 publication of the best seller The Hot Zone, that fear had gone global. The symptoms--catastrophic hemorrhaging, bloody diarrhea and the literal disintegration of one organ after another--were Ebola-like, all right. But the locals needed expert help to make sure. The U.N. organization rushed a team to the scene and, reports Dr. David Heymann, head of the WHO's new Division for Emerging and other Communicable Diseases Surveillance and Control (EMC), "in five days the Ebola diagnosis was confirmed, and the outbreak was nipped in the bud." The medical team had done its job by isolating the victims, providing health-care workers with basic sterile gear, like gowns and gloves, and staying on the scene until no new cases were evident.

If these operations sound more military than medical, there is good reason. Back in the 1960s and '70s, public-health experts felt they had pretty much triumphed over infectious diseases. Smallpox was on the way to extinction; polio was all but vanquished; and, thanks to antibiotics, improving sanitation and pesticides, such maladies as tuberculosis, cholera and malaria were on the run. One by one, humankind's deadliest scourges were being wiped from the earth.

Then came nature's counterattack: in one wave after another, HIV, Ebola, Marburg virus, Lassa fever, Legionnaire's disease, hantavirus, hepatitis C--in all, at least 30 newly identified pathogens over the past two decades--swooped down upon different population groups. Most of them came out of the newly inhabited and exploited rain forests of Africa and South America, making an inter-species jump from animals to humans.