With an estimated 20 million infections a year nationwide, norovirus is the No. 1 cause of the intestinal crud people call stomach flu.

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VANCOUVER, B.C. — Bathing suit? Check.

Suntan lotion? Check.

Nose spray to keep diarrhea from ruining your cruise?

It’s not on the check list yet, but scientists are closing in on a nasal vaccine that would protect against norovirus, the virulent bug that is the curse of cruise ships, cheerleading competitions — and any other venue that brings large numbers of people into proximity.

With an estimated 20 million infections a year nationwide, norovirus is the No. 1 cause of the intestinal crud people call stomach flu.

“This virus is very democratic,” said Jan Vinje, of the U.S. Centers for Disease Control and Prevention (CDC). “It affects everyone.”

If the research goes well, the vaccine could be available within five years, said Charles Arntzen, a molecular biologist at Arizona State University.

“We are going to have a vaccine,” he said last week at the annual meeting of the American Association for the Advancement of Science. But it’s not clear how effective a vaccine would be against a virus that evolves rapidly and comes in more than 30 varieties. And how many people will be willing to get vaccinated for a disease that’s generally just a nuisance?

Sign me up, said Ellen Langan, of Ballard. Langan and her 16-year-old daughter, Ana Krafchick, were among more than 200 people who came down with norovirus after a cheerleading competition in Everett this month. Krafchick started throwing up the next evening and became so dehydrated she passed out and landed in the hospital.

Langan is convinced she picked up the infection while caring for her daughter. “I washed and disinfected my hands until they were raw and I still got it,” she said.

While cruise ships and community outbreaks get most of the publicity, nearly 60 percent of norovirus outbreaks occur in nursing homes, said the CDC’s Vinje. Cruise ships account for 4 percent of reported outbreaks, but the actual fraction is probably lower than 1 percent because many outbreaks are not reported. Another 4 percent of reported outbreaks are linked to schools. Children and adults are equally vulnerable.

Most victims recover after a day or two of misery, but more than 70,000 a year are hospitalized. CDC estimates the virus kills 800 people a year, most of them older than 65. The annual economic toll is about $2 billion in medical costs and lost productivity.

Though it seems like the virus came out of nowhere in the past few decades, it’s been around for a long time, Vinje said. New tests make it easier to diagnose, so tracking has improved. The bug has also changed over time, with new strains emerging every few years.

Illnesses soar when the new strains are more virulent than the old ones. Cases may be spiking again this winter, Vinje said.

As Langan’s experience shows, norovirus spreads quickly and can be harder to kill than the monster in the movie “Alien.” Symptoms hit suddenly. Outbreaks often start when an infected person vomits in the corridor of a cruise ship, or, as in the case of the cheer competition, in a bathroom. Tiny particles fly through air and land on surfaces. Even the simple act of flushing the toilet after a bout of diarrhea or vomiting can suspend more droplets in the air.

The bug can also slip into the body via food, water or dirty hands. Once it does, as few as 18 virus particles are enough to do the trick, making norovirus the most infectious microbe known, Vinje said.

While many viruses are too fragile to survive long in the environment, noroviruses are encased in a BB-like shell that allows them to live for days or even months in some settings. One contaminated airplane cabin spread the disease to successive flight crews over several days.

Cruise ships have learned through hard experience that ordinary mopping isn’t good enough. They now use bleach to disinfect every surface, including hand rails and poker chips, said Dr. Marcia Goldoft, an epidemiologist at the Washington Department of Health.

The first experimental vaccine worked well in a test on 100 people last year, Arntzen said. About two-thirds of those who got the vaccine were protected from infection with one particular strain of norovirus. But a commercial vaccine will need to cover multiple strains, he said. Preferably, it will also be long-lasting, though the bug mutates so quickly that a norovirus vaccine may have to be reformulated every year like the flu vaccine.

A nasal spray is better than a shot because it more directly targets the respiratory tract and gut where the virus concentrates.

Despite a potentially large market, the technical uncertainties have kept big pharmaceutical companies out of the race so far, Arntzen said. His lab at ASU is one of two in the United States working on the problem. The other is Ligocyte Pharmaceuticals, the biotech company that produced the vaccine tested last year.

Nursing homes and health-care workers would probably be the biggest customers, at least initially. But a vaccine might also find a market among cruise-ship clientele or frequent conference-goers.

“It would be more of an insurance policy than a primary health-care protection,” Arntzen said.

In the meantime, since norovirus thumbs its nose at alcohol-based sanitizers, hand-washing remains your best line of personal defense.

But you have to do it right, Goldoft said. That means soap, warm water and vigorous scrubbing for as long as it takes to sing “Happy Birthday” — twice.

A recent study found 83 percent of people say they wash thoroughly, but only 17 percent do.

Sandi Doughton: 206-464-2491 or sdoughton@seattletimes.com