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updated 4/28/2004 2:34:09 PM ET 2004-04-28T18:34:09

It's back -- and it can be very bad. But just how bad will the 2004 return of West Nile virus be?

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In 2002, West Nile virus caused the largest encephalitis epidemic in history. Last years' epidemic was arguably twice as large -- possibly because new tests made it possible to diagnose mild cases of West Nile fever.

The number of serious cases was about the same: 284 deaths in 2002 and 262 deaths in 2003. Thousands of people suffered dangerous brain infections with lingering effects. Many of them -- especially those with what's come to be called West Nile polio -- may never fully recover.

That was one of last year's surprises, says Dr. Grant L. Campbell, Ph.D. Campbell, based in Ft. Collins, Colo., heads the branch of the CDC that keeps track of West Nile cases.

"Last year we saw quite a few cases of acute flaccid paralysis -- the so-called West Nile polio," Campbell tells WebMD. "We have more than 30 such patients here in Colorado. Usually they don't have fever, then -- boom -- they get paralyzed in one limb or another. It often leads to respiratory failure and death. We saw some cases as far back as 1999, but we became more aware of it last year."

Lessons from 2003
The good news, Campbell says, is that the West Nile virus hasn't mutated into a more dangerous form. Although there have been minor changes in the bug's genetic makeup, the virus seen today is still the same virus that made its 1999 American debut in New York City.

Spread by migrating birds -- and perhaps by infected mosquitoes that find their way onto trucks and airplanes -- the virus moved south and then relentlessly marched westward. Last year, the hardest-hit states were Colorado, Nebraska, South Dakota, Texas, and North Dakota.

The virus did move west of the Rocky Mountains, but there were few human infections. Oregon and Washington reported no cases at all. California reported only three.

For reasons nobody can entirely explain, there tends to be a lot more West Nile disease when the virus finds its feet in a new area -- usually the second year after it first appears. It never, ever, goes away. But the number of cases tends to drop off. New York, for example, was where West Nile virus first landed in the U.S. But last year, that state reported only 71 human infections. Colorado, on the other hand, zoomed from 14 cases in 2002 to 2,326 cases in 2003.

"Nobody really understands the flattening out. We've seen it with St. Louis encephalitis virus in years before," Campbell says. "Part of it is bird immunity."

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