The virus, which made its first Western Hemisphere appearance in New York's borough of Queens in 1999, is now considered endemic along most of the East Coast and is making its way west and northward.

Carried by migrating birds, it could become a permanent resident of the entire continental United States.

"It's moving pretty fast," Bob McLean, director of the U.S. Geological Survey's National Wildlife Health Center in Madison, Wisconsin, said in a telephone interview.

"There are eight new states this year and many in the West, including Texas, Nebraska, North Dakota and Manitoba, Canada."

At this time last year only nine states had reported West Nile infections in birds or mosquitoes, but 28 states have done so this year.

Because it is new, some people have reacted with fear, but health officials agree it is not a serious threat to the population.

18 AMERICANS KILLED SINCE 1999

West Nile has killed 18 people since its appearance in the United States in 1999, and caused notable infections in 161, the U.S. Centers for Disease Control and Prevention says.

Two-thirds of the patients have been over 60, many with other illnesses making them susceptible to the infection, which can cause encephalitis or meningitis - inflammations of the brain and spinal cord.

Last year 66 people became ill and nine died. This year so far, the CDC reports 12 cases and no deaths.

Many more people were probably bitten by infected mosquitoes, but in the average healthy person the virus causes no symptoms, or perhaps a vaguely flu-like feeling. Scientists predicted the virus would spread easily.

First reported in Uganda in 1937 and now common across much of Africa, the Middle East and Europe, and the Pacific, West Nile virus was probably carried here by birds.

"You don't like being right," McLean said.

It can only become more entrenched in the United States, he said, because the virus is now found in states such as Louisiana and Florida, where mosquitoes are active year-round, and where the most common species feed heavily on birds.

The virus spreads from mosquitoes to birds, who carry it when they migrate. West Nile is highly deadly to corvids - birds including jays, magpies and crows.

It is less likely to kill thrushes such as robins, or sparrows, pigeons, mourning doves and cardinals, which carry it in their blood, reinfecting mosquitoes that bite them, and then spread it to mammals including people and horses.

"The gulf coastal states look like a source of continual transmission," McLean said. "It also started earlier this year in the northern states, which indicated the virus activity is becoming more endemic - it is occurring all the time."

Unlike many other infections, which rarely are recorded unless a human gets sick enough to go to a hospital, West Nile is easy to monitor. Dead crows are highly visible and can be reported to authorities and tested.

UNFORTUNATE CROWS

"It is fortunate, not for the crows, but fortunate for those of us trying to track disease in that it gives a very good marker for viral activity moving into an area," said Randy Crom of the U.S. Department of Agriculture.

In Washington, D.C., the health department has been overwhelmed with reports of dead crows. Voice mail boxes have filled and residents complain no one comes to test the birds.

"We are finding the first positive birds earlier every year," said Peggy Keller of the capital's animal disease control center. "Last year the first were reported in mid-July and this year it was the first of May."