WASHINGTON -- West Nile virus has sickened 17 people in the United States -- all but one of them in Louisiana -- as it takes a permanent hold in parts of the U.S. southeast, state and federal health officials said on Thursday.

One case was in Mississippi, according to the Centers for Disease Control and Prevention. And a Mississippi health official said there were more suspected cases.

The patients, none of whom has died, range in age from 17 to 88 and several remain hospitalized.

West Nile virus, which has been in the country for three years, has also spread to three new states -- Texas, Oklahoma and North Dakota, the CDC said in its weekly report on death and disease.

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 U.S. east coast, meaning it will come back every year.

West Nile is moving west and north more quickly than experts first predicted. They believe it is carried by migrating birds and infects birds, horses and people.

"It is going to be here to stay. We are going to have to live with it in our midst," said CDC epidemiologist Daniel O'Leary.

People who are infected rarely become seriously ill, he added. "I don't think that people need to be terribly nervous about getting the disease," O'Leary said. "But they do need to take precautions to protect themselves."

These include using repellent, wearing long sleeves and pants outdoors and clearing or treating standing water.

Louisiana State epidemiologist Dr. Raoult Ratard said all of the cases were around Lake Pontchartrain near New Orleans.

"There is a ring there where four to six weeks ago we started having a lot of dead birds, a few dead horses," Ratard said in a telephone interview.

"We warned mosquito control, we warned people about the presence of West Nile virus activity in the area. But there is a huge transmission for some reason that nobody understands."

The 16 people, most of them over 65, developed serious infections that led to meningitis and encephalitis, which are inflammations of the brain and spinal cord.

"Some of them are not in very good shape," Ratard said, adding that he expected there could be some deaths.

---- More Cases Said Likely in Mississippi ---- One case was reported in Mississippi but Dr. Ed Thompson of the Mississippi Department of Health said he expected more.

"We have several more suspect cases," he told Reuters.

West Nile has killed 18 people since its appearance in the United States, and caused serious infections in 161, the CDC said.

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

O'Leary said Louisiana reported one human case of West Nile virus last year. He and other experts noted that states like Louisiana and Florida have what amounts to year-round transmission of West Nile from birds to mosquitoes now.

That does not necessarily translate into a constant risk of human infection, O'Leary said.

"It's hard to predict from year to year when these are going to pop up," he said in a telephone interview from his office in Fort Collins, Colorado.

Mosquitoes must become infected in large numbers and then survive long enough for the virus to build up in their bodies.

Then they must bite people. "All the factors have to be in the same place at the same time," O'Leary said. "Most people who are infected with the virus either never know or experience a mild flu-like illness."

But in some cases the virus, first detected in Uganda in 1937, can cause encephalitis or meningitis.

The CDC did blood tests on residents of Northern Queens in New York after the first U.S outbreak there. They found 2.5 percent of the local population had been infected.

For every 140 infections, only one person developed encephalitis or meningitis. One in five of those infected remember having been sick with a fever.

"Four out of five didn't even know they had been infected," O'Leary said.