HAMILTON -- Western Montana is one of the last bastions against West Nile virus in the United States. But that will change this summer, several infectious disease experts predicted last week at a West Nile virus symposium in Hamilton.

''We expect West Nile to truly arrive here this year," said George Risi, a Missoula physician who is an infectious disease specialist. ''I expect it to be a substantial year for humans."

''The first year, West Nile virus just kind of gets its foot in the door," said Montana State University entomologist Greg Johnson. ''The second year, the gates are open."

The virus arrived in Montana from the East in 2002. Last year, it infected 193 horses and 228 humans, killing four of them. All were east of the Continental Divide except two horses diagnosed late in the summer in Lake County.

Because of the virus's association with mosquitoes and birds, its arrival is inevitable this summer.

''Why are we having a meeting in the Bitterroot about a virus in West Africa?" Marshall Bloom, associate director of the federal Rocky Mountain Laboratories, the symposium's major sponsor, asked about 200 people gathered for the four-hour meeting.

Because, he answered himself, West Nile is among the latest of 30 emerging infectious diseases that have come onto the map since the 1970s. Because West Nile virus is so new to the United States, widespread immunity is not built up. Medicine, science and public health are working to build their knowledge before the first mosquito bite that develops into a West Nile virus fever case. The Hamilton symposium opened with remarks from Gov. Judy Martz and drew specialists from around the state.

West Nile first arrived in the United States in New York in 1999, but it's not a new virus, said Tom Kindt, director of the Division of Intramural Research at the National Institute of Allergy and Infectious Diseases in Bethesda, Md. It was first isolated in 1937 in Uganda in the West Nile River area. During the next 60 years, it caused minor outbreaks and a few deaths in Israel and Africa. West Nile disease in horses was first seen in Egypt and France in the 1960s. But larger outbreaks and serious cases among humans were not seen until the 1990s.

West Nile came to the United States in 1999. It swept across the country, jumped over the Rocky Mountains to California, then spotted back.

While mosquitoes are the vector, or carrier, of the virus, especially the Culex species, it's birds who act as the virus's reservoir. As it goes back and forth between birds and mosquitoes, the virus intensifies.

''The bird is a perfect virus factory," said Jim Murphy, the infectious disease surveillance coordinator for the state at the Montana Department of Public Health and Human Services.

Humans and horses also become ill with the virus, but they're dead-end hosts, in whom the virus does not reach large concentrations.

Birds clearly carry the virus to new areas.

Human cases of West Nile have been documented in every state except Maine, Oregon, Washington, Alaska and Hawaii, Kindt said. It has infected at least 225 species of birds and 49 species of mosquitoes. Crows and blue jays make up 80 percent of the birds infected.

The virus has wildly different outcomes in different people after they're bitten by a virus-carrying mosquito.

Of 100 people infected, 80 will have no symptoms, Murphy said. The other 20 will develop West Nile fever and will have a mild fever that lasts three to 14 days.

But one in 150 people will develop invasive neurological disease.

The incubation period for the illness in humans is three to 14 days after the mosquito bite. West Nile fever symptoms are the sudden onset of fever, malaise, loss of appetite, nausea, vomiting, headaches, eye pains, muscle aches, swollen lymph nodes and skin rash. Most people have symptoms for three to six days and recover.

Serious invasion of the central nervous system occurs 10 times more often in people over 50 and 43 times more often in people over 80. An underlying disease such as diabetes or hypertension can contribute, Risi said.

In Montana, the mosquitoes most likely to carry the virus seem to peak in the second and third weeks of August, said Johnson, the MSU entomologist. He and other scientists trapped mosquitoes in 23 counties last year.

A vaccine for humans is in the works in the labs in Bethesda, but for now the best course is preventing mosquito bites as West Nile visits Montana.