Just a few years ago, a mosquito bite meant nothing more serious than a few days of itching to most Americans. But last summer the mosquito-borne West Nile virus, confined to the East Coast for its first three years in the U.S., swept across the country. More than 4,000 cases of West Nile disease were reported in 39 states in 2002, along with numerous milder cases of the virus. A total of 274 victims died, and many others were left with seemingly permanent disabilities, including a polio-like paralysis, severe muscle tremors, and overwhelming fatigue.

Also in 2002, researchers identified 20 confirmed cases of the virus that had been transmitted through blood transfusions and organ transplants. Blood donors were apparently infected, but not feeling ill, during the brief period before their immune system neutralized the virus.

Spread via a complex cycle involving mosquitoes that bite infected birds and then people, West Nile is now second only to Lyme disease as the most commonly reported insect-borne illness in the U.S. It has also taken aim at wild and domesticated animals, virtually wiping out the crow and blue-jay populations in some areas and, nationwide, killing one-third of the estimated 15,000 horses known to have contracted the disease.

"The explosive epidemics in humans and horses are unequaled in the history of insect-spread illness in the U.S.," said Duane Gubler, director of the Centers for Disease Control and Prevention's division of vector-borne infectious diseases. This summer, the virus could set up shop anywhere in the U.S. or in the hemisphere, Gubler told a February 2003 meeting of public-health officials from across the nation.

From its initial identification in 1937, West Nile virus lived unobtrusively in Asia and Africa, causing a mild, flulike illness. Then, in the late 1990s, it abruptly spread to Eastern Europe. In 1999 it made its American debut in New York.

Based on experience to date, public-health officials estimate that 80 percent of people infected with the virus have no symptoms and never know they've had it. Almost all the rest come down with a syndrome called West Nile fever: a fever accompanied by headache, weakness, and sometimes nausea or a skin rash.

For most people, the syndrome lasts a few days and then they recover, and presumably have long-term immunity to the virus. A small share of those who are infected, about 1 in 150, becomes desperately ill. Doctors know that the risk of getting a really bad case of West Nile rises precipitously with age. Infected children rarely show ill effects. Most who get seriously ill are over 50. People whose immune system has been suppressed by chemotherapy or other causes are also at higher risk.

Beyond that, "we have no idea why one 50-year-old who becomes infected remains asymptomatic while another one is devastated," says Arturo Leis, M.D., a neurologist at Methodist Rehabilitation Center in Jackson, Miss. Leis has cared for many gravely ill West Nile patients.

Severe West Nile disease can take several forms, all of them dangerous. Most of the first known victims had encephalitis or meningitis, an inflammation of the brain or its lining, as well as fever and extreme weakness. So far, about 10 percent of severely affected victims in the U.S. have died; the vast majority of them were in their 70s or 80s.

Last summer, case reports in Louisiana and Mississippi documented the existence of another frightening variety of severe West Nile disease with polio-like symptoms. The patients experienced complete paralysis of one or more limbs, resulting from the virus' destruction of critical motor-neuron cells. But unlike the encephalitis victims, who tended to be in their 70s or 80s, most of the paralytic cases occurred in people in their 50s or sometimes 40s. To date, none have recovered. "The prognosis is very similar to polio," Dr. Leis says. "All the patients remain profoundly weak."

Dead birds are often the first clue that the virus is in the neighborhood. West Nile "is really a bird disease that extends to people through infected mosquitoes," says Lyle Petersen, M.D., deputy director of the CDC's division of vector-borne infectious diseases. When the weather warms enough to prompt infected female mosquitoes to wake up from hibernation, they start hunting for the blood meals they need to lay eggs. They bite birds and infect them with the virus; the birds then pass the virus on to other mosquitoes that bite them.

"The whole cycle escalates, and by the late summer, there are so many birds and mosquitoes infected that human outbreaks occur," Petersen says.

The hardest-hit birds are the corvids: crows, magpies, jays, and ravens. "In the New York City area, by August or September 1999 and 2000, over 90 percent of dead crows turned in for testing were positive," says Robert G. McLean, Ph.D., program manager for wildlife diseases at the U.S. Department of Agriculture's National Wildlife Research Center in Fort Collins, Colo. "It's unreal; I've never seen a virus that severe."

New CDC research has shown that birds can also get infected just by sharing a roost with other infected birds or by eating infected birds and other animals. It's important not to touch any dead bird or animal you encounter.

As for mosquitoes, biologists are confident they've fingered the major species carrying the West Nile virus: the closely related regional members of the genus Culex. In the Northeast, it's Culex pipiens, the northern house mosquito; in the Southeast, Culex quinquefasciatas; and in the West, Culex tarsalis.

"They're called house mosquitoes for a reason," Petersen says. "They like to hang out in sheltered areas like houses and live quite happily in an urban environment." Culex are readily infected by the virus. And they spread the disease because they bite birds, people, and other animals.

The key to keeping mosquitoes out of your house and yard is denying the females comfortable places to lay their eggs. It takes energy for mosquitoes to fly. If you're breeding them in your yard, you're the first one they're going to bite, say mosquito-control experts.

House mosquitoes are drawn to muddy, murky bodies of water. Keep your yard and house free of containers filled with standing water and you've won half the battle.

Keep your rain gutters scrupulously clean; Culex mosquitoes are partial to waterlogged, decaying leaves.

The most effective way to avoid bites is to stay inside when mosquitoes are active. For Culex and most mosquitoes, that means at dusk and throughout the evening.

If you must go out then, wear light-colored, loose-fitting clothing that covers as much of your skin as comfort permits. Mosquitoes will bite right through tight-fitting fabrics. Avoid perfumes and aftershaves, which attract some mosquitoes.

Mosquito spraying is strictly a local affair. Some regions, typically ones that have longstanding mosquito problems, have permanent areawide mosquito-control programs. In other places, including large cities such as Chicago, Cleveland, and New York, the job of mosquito control falls on the local health department. No matter who's in charge, they have broad discretion on how to go about the job. In so doing, they must balance the danger of infection with West Nile against the risks of spraying, such as potential harm to health and the environment.

Regardless of local conditions, however, all good mosquito-spraying programs have certain things in common:

The choice of when and where to spray should be based on constant surveillance and monitoring of West Nile in birds, mosquitoes, and people. Ideally, spraying should target populations of mosquitoes after they have tested positive for the virus but before they have infected any people.

Residents should get plenty of warning about when and where spraying will take place, and which pesticides will be used. During spraying and for several hours afterward, people should stay inside with their windows and doors closed to allow time for the pesticides to disperse and break down. Families should bring outdoor toys inside before the spraying. Asthmatics should be especially cautious, since the sprays can set off asthma attacks in susceptible people.

Spraying should use the least-toxic pesticides. In most cases, the preferred choice is one of the synthetic pyrethroids, fast-acting compounds that break down quickly once they're sprayed. They are thousands of times more toxic to insects than to people (though they are toxic to fish and bees). Some widely used products in this category include permethrin, resmethrin, and sumithrin.

Last September 2002, scientists from the CDC's National Center for Environmental Health conducted a study of 193 people who lived in areas of Mississippi that had been sprayed for mosquito control with people who lived elsewhere. The researchers measured the amount of pyrethroid pesticide-breakdown products in the test subjects' urine samples, and they found no difference in levels between the two groups.