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Spring showers this year have produced a bumper crop of mosquitoes — and heightened fears of the West Nile virus they carry. So far this year, the virus is off to a relatively slow start (the first human cases began showing up last week), but experts predict that by the time it peaks in August, this year's West Nile season may be the worst yet — worse even than that of 2002, when 4,000 Americans were infected and 284 died.

So how much do we know about West Nile? A lot more than we did, thanks to a study published in the current issue of the journal Neurology. A detailed analysis of 23 cases conducted by researchers at the Cleveland Clinic found symptoms that they weren't expecting and didn't find some that they were. The skin rash thought to be the hallmark of the virus showed up in only about 25% of cases. On the other hand, nearly half the patients studied experienced significant muscle weakness or partial paralysis. In fact, paralysis — up to and including generalized quadriplegia — was so common that doctors are starting to compare West Nile to the last major epidemic of infectious paralysis: the polio outbreak of the 1950s.

In other respects, the virus seems to be behaving as expected. Most people who get infected don't get sick enough to end up in a study; the vast majority (nearly 99%) feel hardly anything at all. When there are symptoms, they are almost always accompanied by a fever and about half the time by some gastrointestinal distress. Unless the disease progresses to cause encephalitis or meningitis, it's usually over in less than a week.

There is no effective treatment, but the prevention strategies used to avoid regular mosquito bites work pretty well: wearing insect repellent with 35% DEET (10% for kids), especially at dawn and dusk, when the insects are most active, and draining those pools of standing water, where the bugs like to breed.

Dr. Gupta is a neurosurgeon and CNN medical correspondent

• Sanjay Gupta, M.D.