West Nile virus was long considered a serious problem only for the elderly and frail, and more of a nuisance illness for everyone else. Now a surprising number of patients show the virus is more threatening than widely believed -- and new research finds that even so-called mild cases of West Nile fever can impair people for weeks or months.

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It's sobering news as the nation gears up for a seventh season of the mosquito-borne virus.

"West Nile is fading a little bit from the public consciousness," worries Dr. Henry Masur of the National Institutes of Health. "Still, there are more cases of paralysis (from West Nile) than there were in many years of polio."

West Nile virus had stricken abroad for decades, from the tip of Africa up to Europe and throughout Asia, before it appeared in New York City in 1999 and began an inexorable march across this country. Since then, the Centers for Disease Control and Prevention have counted more than 16,600 human cases and 654 deaths.

Severe illness still is rare, considering that 80 percent of people infected never show symptoms.

But last year, about a third of the West Nile cases reported to CDC had neurologic complications like meningitis or encephalitis. Those are most common in older adults.

Then there are patients who came down with West Nile's most perplexing complication: polio-like paralysis or severe muscle weakness that often strikes healthy people in their 30s, 40s and 50s. They may show no other symptoms before a limb suddenly quits working. Sometimes, the paralysis leads to respiratory failure.

There are no good counts of this West Nile complication, and some doctors believe it's frequently mistaken for a stroke or other paralyzing ailment.

In a study to be published in July, CDC's Dr. Jim Sejvar estimates that 10 percent of people who develop the most severe West Nile disease may have some degree of the polio-like complication, and many don't recover muscle function.

Even the less serious form of illness, West Nile fever, is turning out to be harder to kick than doctors initially described, so much so that the CDC has largely abandoned its earlier characterization as a "mild disease."

A study by Chicago's health department last fall found that West Nile fever was bad enough to keep half of sufferers out of school or work for 10 days, fatigue lasted a month -- and the median time to get back to normal was a stunning 60 days.

What's happening? In much of the world, West Nile is a fairly mild illness. But the form working its way through the United States appears similar to a more virulent Israeli strain, something not initially apparent to health workers.

"There was no recognition that it could paralyze and kill people that were healthy and relatively young," says Heller, now 56, a physician's assistant who shared rehabilitation with a 40-year-old also paralyzed in late 2003. "We were really caught off guard."

There is no vaccine yet, nor is there an approved West Nile treatment. The best protection is to avoid mosquitoes, using repellent whenever you're outdoors and not letting puddles collect in flower pots, wading pools or other spots where mosquitoes can breed.

Anyone with symptoms of serious illness should see a doctor right away: high fever, severe headache, confusion or difficulty thinking, stiff neck, severe muscle weakness, or tremors.

Studies of potential West Nile therapies -- including infusions of West Nile-fighting antibodies from the blood of survivors -- are poised to begin as soon as this year's first patients appear. California especially is putting doctors and residents on notice to seek out these research studies, as that state braces for what it expects to be a large outbreak.

"If new symptoms develop at any age, don't assume that it is an innocuous, transient viral infection," says Dr. Patrick Joseph, a San Francisco physician and member of the National Foundation of Infectious Diseases. "Seek help so diagnostic testing can be done."

For those who suffer paralysis, prompt physical therapy is crucial, too, advises Dr. Mazen Dimachkie of the University of Texas Medical School in Houston.

So far this season, no human cases of West Nile have been reported in the Houston area. However, Harris County Mosquito Control officials warn the season has just begun. They are already spraying in areas where birds and mosquitoes have tested positive for the disease.