Since the beginning of October, Boston has seen an increase in cases of gastrointestinal illness, including nine clusters at local schools, day-care centers, and a hospital, the Boston Public Health Commission said yesterday.

For the week ended Dec. 18, complaints of vomiting or diarrhea accounted for 2.9 percent of all emergency room visits in the city, compared with 2 percent for the same week in 2009.

The illnesses have been linked to norovirus, a common and easily spread viral infection that causes severe nausea, vomiting, and diarrhea. it usually resolves on its own in about two days, said Dr. Anita Barry, director of the infectious disease bureau at the health commission.

She said samples from two clusters that were tested by the federal government confirmed the presence of the virus. It’s often referred to as the “cruise ship’’ virus because it has been known to sicken seaborne tourists.

This time it is staying on land, though it is tough to say how many folks in this region have come down with the stomach bug. the health commission counts 2,251 people who were seen in Boston emergency departments from October through Dec. 18. An increase of 0.9 percent translates into 20 extra people going to a hospital over that period.

While you can get infected with norovirus by eating contaminated food, the virus typically is not the source of a single foodborne illness outbreak like the salmonella outbreak earlier this year in eggs, Barry said.

Norovirus often spreads from person to person because of poor hygiene such as not washing hands properly after going to the bathroom or changing an infant’s diaper.

You also can also remain contagious for up to three days after your symptoms disappear. “That’s why food handlers have to stay out of work for 72 hours after they’ve been sick,’’ Barry said.

People who come down with an intestinal bug, Barry said, should drink plenty of fluids to stay hydrated; take ibuprofen or acetaminophen to reduce any fever; wash their hands frequently to avoid infecting family members; and avoid food preparation while sick and for three days afterward.

If the illness doesn’t resolve after a few days or you start to feel dizzy or faint, call a doctor.

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