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People over 50 are more likely to have complications from West Nile virus, the mosquito-borne infection that is rapidly spreading across the United States, but doctors say severe cases are the exception.

"Nothing happens to most people infected by the virus," says Steven Opal, M.D., chief of infectious disease at Memorial Hospital in Providence, R.I. "But the older you are, the more likely you are to become ill."

Last year, nearly 4,000 Americans infected with the virus were sick enough to seek medical attention. Of 250 people who died, three of four were over age 60. Scientists predict a worse outbreak in 2003.

"We're expecting it all over the country, and in much greater numbers than last year," says Robert Haley, M.D., chief of epidemiology at the University of Texas-Southwestern Medical Center at Dallas.

"Where there are mosquitoes, there's danger of West Nile virus."

By the end of 2002—three years after the virus first appeared in the United States—human cases of West Nile had been detected in 39 states and the District of Columbia. By this summer, they're expected to turn up in most of the remaining states, including Arizona, Idaho, Maine, New Hampshire, Nevada, New Mexico, Oregon, Utah and Washington.

States may get help in controlling the disease through $100 million in grants the House approved last month in the Mosquito Abatement for Safety and Health Act.

The virus is transmitted to people and some animals (horses, cats, bats, domestic rabbits, squirrels) by a variety of mosquitoes that feed on infected birds. There is no evidence, says the U.S. Centers for Disease Control and Prevention (CDC), that the disease can be spread from human to human or to a human by any animal except a mosquito.

"As a rule, as long as you're not bitten by a mosquito, you cannot get it," says Charles Ericsson, M.D., chief of clinical infectious diseases at the University of Texas-Houston.

There have been isolated reports of West Nile virus transmitted via blood transfusion and organ transplant. But Lyle Petersen, M.D., CDC's deputy director for science of vector-borne infectious diseases, says, "If you need blood products, West Nile virus should be very low on your list of concerns. The amount of contamination remains relatively small."

Most people who are infected will not get sick; about 20 percent will have mild symptoms, such as chills, muscle aches, slight headache and low fever. There is no treatment for the virus, but most cases clear up on their own after a few days.

Why older people are more apt to get sick is not clear. Infectious disease expert David Pegues, M.D., of the University of California at Los Angeles School of Medicine says as their immune systems "change and age, it's possible they respond inadequately to certain infections."

The most serious complication of the virus is West Nile encephalitis, brain inflammation accompanied by very high fever, severe headache, disorientation, convulsions, double vision and even paralysis or coma.

West Nile encephalitis usually clears up after several days or weeks. Very ill patients may need hospitalization for "supportive" care (such as intravenous fluids, respiratory assistance and therapies to prevent secondary infections).

With no cure at hand, researchers are testing several vaccines to prevent infection. "A dozen compounds have some effectiveness at cell level," says Patricia Repik, program officer for emerging viral diseases at the National Institute of Allergy and Infectious Diseases.

If an experimental vaccine from Acambis, a pharmaceutical firm in Cambridge, Mass., proves ultimately to be effective and safe, it could be available in four to five years.

Other studies show that the antiviral drug ribavirin may ease severe symptoms. "It's encouraging," Pegues says.

So is the notion that the threat of West Nile may fade as we build up our immunity to the virus. Indeed, in countries such as Egypt, which gave the virus its name, the disease is common, but people rarely get ill.

"Eventually the birds and mosquitoes will become relatively resistant, and the virus will decrease in frequency," Haley of the University of Texas says. "This is what usually happens. The viruses come in, create havoc for several years and then decline."

Michele Meyer is a freelance writer in Texas.