Cases of norovirus — the virulent, extremely contagious virus that causes vomiting and diarrhea — are increasing in the U.S., the Centers for Disease Control says, reporting twice as many cases as this time last year.

The CDC reported 91 outbreaks of the virus for the week of Dec. 5, up from 69 the previous week — and more than double the 41 outbreaks from the previous year. And as of Dec. 23, more than 40 outbreaks of the virus were reported by the Minnesota Department of Health.

Although it is commonly called the stomach flu because of the common symptoms, norovirus is not caused by the influenza virus. “You can become infected with norovirus by accidentally getting tiny particles of feces (feces) or vomit from a norovirus-infected person in your mouth,” the CDC says. The reason it’s so contagious is because when you’re sick, you can shed “billions of norovirus particles that you can’t see without a microscope,” the agency explains.

“It only takes a few norovirus particles to make you and other people sick.”

The virus causes acute gastroenteritis – an inflammation of the stomach or intestines – which causes diarrhea, vomiting and stomach pain. Although most people recover in three days or less, the CDC says someone who has had norovirus can still be contagious for days after they recover.

It was the cause of a recent outbreak in California in which at least 80 people became ill from raw oysters served at an event. And in Hawaii, the virus led to the closure of a popular hiking area after dozens of campers fell ill with the virus.

To limit the risk of contracting the virus, the CDC recommends handwashing — but notes that hand sanitizers do not work well against the virus. The agency also recommends washing fruits and vegetables thoroughly before eating and cooking oysters and other shellfish to an internal temperature of at least 145°F. This is the temperature at which the heat-resistant norovirus eventually begins to die.

Although there is no specific treatment for norovirus, the Mayo Clinic recommends staying hydrated. In severe cases, medical intervention for intravenous rehydration may be necessary.

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