An illustration of a stomach against a background of a tumultuous sea.

If you’ve ever been hit with a few days of intense nausea and vomiting, there’s a good chance that the culprit was a bug called norovirus, which sickens between 19 and 21 million people in the U.S. every year.

Sometimes mistakenly called the stomach flu, this virus—which can also cause diarrhea, stomach pain, fever, and headache or body aches—gets a lot of attention for making swaths of tourists and workers sick on cruise ships. The Centers for Disease Control and Prevention reports that it has caused outbreaks on two cruise lines so far in 2019, sickening more than 600 people.

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But you don’t have to embark on a Caribbean vacation to catch norovirus. The bug is ubiquitous, says Amesh Adalja, M.D., an infectious-disease physician and senior scholar at the Johns Hopkins University Center for Health Security and a spokesperson for the Infectious Diseases Society of America. It can spread quickly in any spot where a lot of people congregate, such as a convention, or in restaurants, healthcare settings such as nursing homes, and schools and daycares.

“You can get norovirus anywhere, and the majority of virus is not on cruise ships,” he says. (In fact, cruises account for only 1 percent of norovirus outbreaks worldwide, according to the CDC.) Outbreaks are most common from November to April, but you can contract it at any time.

And while most people recover in a few days, the vomiting and diarrhea can cause more serious problems. “The major danger is dehydration,” says Karen Kotloff, M.D., professor of pediatrics and medicine and head of the division of infectious disease and tropical pediatrics at the University of Maryland School of Medicine in Baltimore. Young children, older adults, and people with other chronic conditions, such as kidney disease or diabetes, are more likely to become dehydrated.

Here, how to protect yourself from norovirus and what to do if the bug strikes.

Norovirus Is Easy to Catch

Norovirus is the leading cause of foodborne illness in the U.S. Contamination often occurs during food preparation, says Juan Leon, Ph.D., M.P.H., associate professor of global health at Emory University in Atlanta.

Here’s how: Food handlers at restaurants or cafeterias may have the virus on their hands—probably because of improper hand-washing after using the bathroom—and transfer it to food. The same thing can happen when people prepare food at home.

But you can also catch norovirus in other ways. The virus, experts say, is extremely hardy, meaning it can persist in the environment for long periods of time. It can live on surfaces such as doorknobs and handrails for weeks, where you can unwittingly pick it up.

And it may last even longer in water. One study, published in the journal Applied and Environmental Microbiology, employed 13 brave volunteers who drank samples of norovirus-contaminated water that sat for various periods of time.

The two volunteers who drank water that had been spiked with norovirus 61 days beforehand both got sick, which meant the virus remained active and infectious in water for at least two months.

Additionally, it takes only a tiny amount of the virus to make you sick. So even trace amounts of contamination can cause an infection.

Keep From Getting Sick

There’s no vaccine against norovirus, though researchers are working on developing one. It’s a challenge, Kotloff says, because norovirus, like influenza, mutates frequently—so any vaccine would need to continue protecting against the virus even as it changes.

Right now, diligent hand-washing, with soap and water, is the most effective way to prevent norovirus. Alcohol-based hand sanitizer alone won’t protect you because alcohol won’t kill norovirus.

In the kitchen, use proper food safety practices, such as thoroughly washing produce and cooking foods adequately. Shellfish is a common source of norovirus outbreaks, so it’s best not to eat it raw. And Leon advises peeling produce when possible, rather than eating it with the skin on.

If you or someone in your family has just had norovirus, take care to keep common spaces clean. Opt for bleach-based cleaning products, or a bleach solution of 5 to 25 tablespoons of household bleach in 1 gallon of water, according to the CDC. Use it to swab areas where food is being prepared, and to clean the bathroom and any other spots where someone vomited recently.

And ban anyone who has norovirus, or is recovering from it, from food-preparation areas. The CDC recommends that people wait at least two days after their symptoms resolve before making food for anyone else.

How to Handle an Infection

Because there’s no treatment for norovirus—antibiotics won’t work against it—your main goal should be staying hydrated.

That can be tricky because you may be vomiting—and losing fluid—frequently, Kotloff says. Rather than drinking a lot at one time, she recommends smaller, frequent sips of fluid, which may allow your body to absorb the liquid before you can throw it up. Sports drinks and broths are good options.

Eat when you feel like it. Small meals are less likely to trigger vomiting than large ones, and opt for bland, easily digested foods such as saltine crackers, soups, potatoes, rice, bananas, and cooked vegetables. Avoid foods that are fatty or spicy, Kotloff says. For young children, oral rehydration products, such as Pedialyte, are better choices than sugary drinks.

People who are at risk for dehydration may want to call their doctors when they first get sick, to ask about a prescription for an anti-nausea and vomiting medication, such as ondansetron (Zofram and generic), Adalja says. That can help you keep foods and fluids down.

And be on the lookout for signs of severe dehydration. In very young children, these can include decreased urination, crying without any tears, dry mouth, sunken eyes, cool or clammy hands and feet, and listlessness or fussiness. In adults, signs of an emergency can include not urinating or dark colored urine, irritability, confusion, dizziness, lightheadedness, rapid heartbeat or breathing, sunken eyes, listlessness, unconsciousness, or delirium.

If you suspect severe dehydration in yourself or your child, get to an emergency room. There, you can get fluids through an IV, rather than orally.