Poised atop a single water-ski, Richelle Matli sliced the waves around floating buoys to win second place in a slalom event in Fort Collins, Colo., in July 2003. In peak form, the 47 year-old mother of two teenagers returned to Pleasanton, Calif., eager to train for the national championchips. Instead, two weeks later, Ms. Matli lay paralyzed in a hospital, unable to walk.

What plunged an elite athlete from the pinnacle of her sport into paralysis was an inperceptable pinprick: the bite of a mosquito carrying the West Nile virus. The infection first ignited fever, aches and profound fatigue, and then invaded her spinal cord. Her strong left leg turned into a dead weight. Some doctors feared that she would never walk again. After months of therapy, mobility returned.

Ms. Matli is one of thousands of patients who have developed serious neurological complications from the West Nile virus. Working its way inexorably across the country the past five years, West Nile has upended early assumptions that it was a mild disease that generally only posed a serious threat to the frail elderly.

U. S. scientists and public-health officials are now concluding that West Nile is a far more nimble and virulent foe than they had first thought. Since arriving in New York in 1999, an aggressive mutant form has hopscotched a step ahead of scientists and has exploited shortcomings in the public-health system. Along its way, West Nile has assailed a surprising number of healthy adults in their 30s, 40s and 50s, causing complications such as meningitis, encephalitis and paralysis, which can leave victims physically disabled, brain-damaged or dead.

Better data, along with new studies of West Nile patients and the virus itself, are just now helping researchers and health professionals understand what they're up against. Still a vaccine or treatment appears far off. And even with improved information, the public-health infrastructure--faces big challenges to overtake this widening epidemic.

Amid growing concerns about severe acute respiratory syndrome, bird-flu and other infections that spread rapidly around the globe, West Nile's swift infiltration here reveals this country's vulnerabilities to dangerous imported diseases.

The U. S. faces a "second poliomyelitis" until a vaccine or drug is found, says James Rahal, a scientist researching treatments at Cornell University's Weill College of Medicine, referring to polio, the paralyzing disease eliminated in developed countries decades ago thanks to vaccines.

West Nile, largely a summertime illness, has sickened over 16,000 Americans, and caused more than 600 deaths and more than 6,500 cases of severe neurological disease since 1999. This year has seen human cases in 40 states, with California, Arizona and Colorado hit the hardest.

Health officials still believe that most people exposed to the virus don't have symptoms. But this year, 37% of West Nile patients whose cases were reported to the government developed "neuroinvasive" complications, like meningitis, encephalitis and paralysis.

So far in 2004, there have been a total of 1,951 cases of West Nile disease overall including 720 cases or about 37% with neurological symptoms, and 62 deaths. In 2003, there were a total of 9,862 cases, including 2,866 or about 29% with neurological symptoms and 264 deaths.

Even the less serious form of the disease, West Nile fever, is turning out to be more debilitating than previously realized. A study in the September issue of the Annals of Internal Medicine found the median recovery time was 60 days, longer than previously recognized. (Previously, it was thought that overall cases, especially West Nile fever rebounded much more quickly.) In another recent study, the New York City Department of Health and the U. S. Centers for Disease Control and Prevention found that only 37% of New York patients with West Nile disease recovered completely after one year.

Venturing out to his mailbox without mosquito repellent once in July 2003, Lyle Petersen, an expert in West Nile virus at the CDC in Fort Collins, himself contracted a case of West Nile fever that left him bed-bound for five days and profoundly fatigued for weeks. "I was sick as a dog," he recalls. "After that, I stopped calling it a mild disease."

West Nile has managed to confound observers in part by eluding data trackers. While West Nile encephalitis, a brain infection, was made a reportable disease by the CDC in 2001, West Nile paralysis and fever weren't added to the list until 2003.

Not everyone who runs a temperature goes to their doctor or has a blood test to determine the cause of fever. And ultimately, reporting of West Nile fever to the federal government is left up to the states, as well as individual doctors' discretion. The setup virtually ensures that the full spectrum of disease remains undercounted. The first two major studies about the disease's long recovery time and the disabilities that linger--fatigue, difficulty walking, and balance problems--just came out in the past two months.

Recent improvements in the data are starting to draw public attention to the disease. "The evidence clearly says the science hasn't been adequate. Otherwise we wouldn't have 600 people dead," says U. S. Rep. Doug Ose, Republican of California, chairman of the House Government Reform Subcommittee on Energy Policy, Natural Resources and Regulatory Affairs. Mr. Ose chaired a recent hearing to examine potential gaps in public health and pest control.

Still, a concerted federal assault on mosquitoes is unlikely. The CDC is currently updating its guidance on mosquito abatement, but the federal agency doesn't exercise authority over local mosquito-abatement districts. A federal bill to give matching funds to localities for West Nile spraying was passed by Congress in 2003 but the money hasn't been appropriated.

Localities face their own challenges. Spraying to kill mosquitoes is controversial. Recent appeals court decisions in the West, not directly related to West Nile, have favored environmental brakes on pesticide applications. A Ninth Circuit Court of Appeals 2001 decision held that even spraying that complies with pesticide laws requires a Clean Water Act permit under the National Pollution Discharge Elimination System.

It can also be hard to muster public enthusiasm for preventive measures. In California and Arizona, states bearing the brunt of the virus this year, nearly two-thirds of people surveyed said they hadn't taken precautions against mosquito bites, according to a telephone survey conducted in August by the Harvard School of Public Health. Just half of Americans overall know what DEET is, the study said. (It's the active ingredient in the CDC-recommended insect repellent.)

As for municipal spraying, while the Harvard study suggests majorities support it, authorities often contend with complaints that people find it inconvenient, unpleasant and unsettling to live amid a chemical cloud. Some localities have tried milder measures, such as emptying ponds and placing biological pesticides in drains to arrest development of mosquito larva so they don't sprout wings to fly.

The agile virus has also outpaced a research bureaucracy bound to follow rigid rules for conducting studies. Drug researchers say they select test sites based on their best guess as to where cases will occur. They go on to get ethics reviews by area hospitals' institutional review boards, only to have the outbreak number occur somewhere else. By the time they climb the mountain of paperwork for a new locale, an outbreak may be over. To defeat fast epidemics, says the CDC's Dr. Petersen, "We need a more flexible system."

Currently, there is no approved treatment for West Nile. Drug research at Cornell University will test a blend of the natural antiviral interferon, and tests at the National Institutes of Health will test a treatment called intravenous immune globulin. Experimental vaccines also are being pursued, but they are years away. The only preventive immunization now in use is a veterinary vaccine to protect horses.

For now, personal protection remains the best means of prevention. Among products available are sprays with repellent DEET for the skin, clothing treatments containing the insecticide permethrin, and bed nets for camping. But even when used, these are an imperfect barrier, CDC scientists say.

Shawn Richards, 36, says he diligently applied DEET while on a family camping trip to eastern Colorado in August last year with his wife and children. But he felt danger was remote. "We were like everyone else. I said it's not going to happen to me," he recalls.

A week later, a stiff sore neck sent the land surveyor to the chiropractor. Headache and blurred vision set in. His doctor suggested Tylenol, but also drew a blood sample to test for West Nile. While he awaited test results, Mr. Richards's condition deteriorated. At his parents' urging, he went to the emergency room. A spinal tap confirmed that he had meningitis, and tests confirmed that the meningitis was due to West Nile.

"My temperature reached a high of 105.6. My brain was cooking and I didn't even realize it," he wrote in his journal. "My thoughts were very scattered and it was hard to think." He lost 20 pounds. The eventual diagnosis was both meningitis, inflamation of the brain's outer coating, producing headaches, and encephalitis, an infection of the brain that clouded his thinking and damaged his short term memory.

He was discharged after about a week when his fever subsided, but headaches, blurred vision, weakness and confusion persisted. Routine tasks like finding his lunch or coffee stumped him. Figuring out his truck's automatic ignition mystified him one day for about 15 minutes. Walking his kids to and from the bus stop drained him. Unable to read or do much of anything besides watch cooking shows, he grew depressed and began taking antidepressants. He consulted a neurologist.

In mental status exams, he strained to recall strings of words like cat, yellow, sky” or count backwards from 100 by sevens. The doctor offered an Alzheimer’s drug, which upset his stomach. Within days, he dropped the drug, choosing to exercise his brain by memorizing lists and names while driving his car.

After seven weeks and a partial recovery, Mr. Richards had exhausted his sick leave and short-term disability. Fearful of losing his livelihood, he returned to his job. Now, he’s survived encephalitis ans feels 95% back to normal. It has made him conservative.

”Before we took precautions but didn’t let it change our lives,” he says. “Now I’ve changed. If I see one mosquito, I push the kids inside or spray them down.”

The initial view of West Nile as a relatively mild disease was based on old historical models of the disease in Africa. West Nile virus, first identified in 1937 in Uganda, often involved only three to five days of low-grade fever. For decades, it ran a mostly mild course through South Africa and Egypt. Relatively innocuous and believed to target tumor cells, the virus was even tested as a cancer therapy in the mid-1950’s.

But as West Nile virus spread into the Middle East and Europe, something changed. When an outbreak hit Romania in 1996, doctors reported a jump in once-rare neurological complications such as coma and paralysis. This pumped-up virus triggered more outbreaks in Russia and Israel.

By 1999, an imported West Nile virus closely resembling the virulent Israeli strain arrived in New York. Early on, scientists knew it was a “hot” strain, says Ian Lipkin, an expert in viruses at Columbia University Medical Center who identified West Nile virus in New York encephalitis patients in 1999. But it’s only now that they’re starting to understand why.

Researcher David Beasley and a team at the University of Texas Medical Branch in Galveston compared genes on the New York strain with 19 other strains of West Nile virus. He found a mutation on the New York virus suspected of boosting virulence. Genetics studies could help lead to a vaccine.

When the super-virulent strain landed in the U.S., saturating swarms more deeply that does St. Louis encephalitis, a similar brain infection. Most surprisingly, the disease cropped up in places without past mosquito problems--and hence, without the pest-control programs needed to stop them.

Richelle Matli, chief radiological technician at California Pacific Medical Center in San Francisco, ran a busy X-ray suite when she wasn't jumping wakes and winning water-skiing titles. On the job, she props up frail patients and holds down squirming toddlers to ensure a clear X-ray picture.

Even in her white lab coat, she wears a gold water-ski pendant, an emblem of the sport where she ranked fifth in the nation in the slalom--until last year, when she made a trip to regional championships in Fort Collins. The lake seemed mosquito-free, but an outdoor banquet site was swarming. She remembers joking that everyone would get West Nile, though she found no obvious bites. She took home second place and a chance at the national championships in Houston.

Back in California, while training and packing for the nationals, she developed a rash, diarrhea and crushing fatigue. She at first chalked it up to the heat, exhaustion and parties. But soon, she had trouble walking. Neurologists probed her left leg, finding flaccid muscles and absent reflexes. Worsening weakness left the leg limp. Admitted to intensive care at the University of California at San Francisco, she underwent tests that confirmed West Nile.

The imminent threat of West Nile paralysis is that it can ascend to immobilize the lungs, requiring patients to be put on a ventilator. Respiratory failure is the cause of many West Nile deaths. Ms. Matli continued to breathe on her own. But after three weeks in the hospital, her paralysis persisted. Discharged to rehabilitation, she was reduced to navigating her world with a walker.

"I had to fight the complete unwillingness of my muscles to do anything. ...It wasn't pretty," says Ms. Matli. She went to physical therapy three to four times a week, for four to five hours each day, for five months. When her insurance ran out, she overcame her distaste for gyms, and pushed her inert limb through pool therapy, weight training and cycling. Mobility returned slowly.

"It took me three months to raise my foot the height of a stair," she says.

Little is known about the prognosis for West Nile paralysis, says neuro-epidemiologist James Sejvar of the CDC. He is now running a research study of 32 Coloradoans paralyzed last year to better predict who will recover, and who will be irreversibly damaged.

After eight months, Ms. Matli went back to work at the X-ray lab but still limps. She reached a milestone when she got up on skis in February. This time, there were no prizes. She had to rework her left ski binding to fit her still clenched toes. She made three attempts just to stand up. But a crowd of friends got up at dawn to cheer her return to the sport. "I've still got a long way to go," she says.

For more information of West Nile virus, click on the link below to go to the Centers for Disease Control and Prevention website.

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