A way to test the nation's blood supply against the virus might be in place next summer, the scientists said Tuesday.

The mosquito-borne virus has infected 2,000 people in 32 states so far this year and killed 98. Particularly worrisome are recent discoveries that West Nile apparently can be spread through blood transfusions if someone donates blood shortly after becoming infected, and that it occasionally causes a polio ( news - web sites)-like paralysis.

Still, public health specialists are expressing cautious optimism. While West Nile virus is here to stay, they expect infections to fall sharply — possibly as early as next year — as more people become immune and communities act quickly each spring to destroy mosquito eggs and breeding grounds.

There even are hints that immunity against cousins of West Nile — such as yellow fever, dengue fever and St. Louis encephalitis — might provide protection against the new virus too.

In experiments, genes from the West Nile virus were added to the structure of the yellow fever vaccine, and proved protective in animal studies, said Dr. Anthony Fauci of the National Institutes of Health ( news - web sites).

A biotechnology company, Acambis, plans to begin tests in a few dozen people soon to see if the experimental vaccine is safe, and if later testing proves its effectiveness, it could be available in three years, Fauci told a Senate hearing.

Also, the NIH this winter will test people vaccinated against yellow fever — inoculations required for travel to certain parts of Africa and South America — for signs that it provided cross-protection against West Nile.

"This is a disease we need to take seriously," Fauci told senators who wondered why West Nile, which first hit the United States in 1999, spread so fast. But "it's extraordinarily unlikely that West Nile will ever get on the same radar screen" as, say, flu epidemics in terms of the harm it causes, he said.

For one thing, only 20 percent of people who become infected with West Nile have even mild flulike symptoms; one in 150 to 200 get seriously ill. Better, if you survive a West Nile infection — even if you had no symptoms — you're considered immune for life.

So today's children and young adults probably will become immune by the time they reach 60, important because older people are most at risk of death from West Nile, Fauci explained.

The Food and Drug Administration ( news - web sites) hopes to have blood banks testing for West Nile virus in donations — even if it means using an experimental test — next summer, said FDA's Dr. Jesse Goodman.

While West Nile apparently can be transmitted through blood, the virus stays in the bloodstream for only a short time — suggesting the risk to the blood supply is very low, he said. Still, a blood-testing technology called nucleic acid testing can detect low levels of other viruses in blood, and FDA is optimistic that tweaking that technology eventually could allow West Nile detection too.

Meanwhile, mosquitoes don't disappear during the winter in the South. So people should continue to wear DEET-containing mosquito-repellant and should not let water stand in flower pots or buckets near their homes, stressed Dr. Julie Gerberding, director of the Centers for Disease Control and Prevention ( news - web sites).

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On the Net:

West Nile virus site: http://www.cdc.gov/ncidod/dvbid/westnile/index.htm