Cases of “tripledemic” viruses like flu, RSV, and COVID appear to be holding steady or declining in many parts of the U.S. But rates of another miserable—and potentially dangerous—virus are on the rise.

Respiratory viruses like RSV and flu got off to “early and brisk” starts this winter season, Dr. William Schaffner, professor of infectious diseases at Vanderbilt University Medical Center in Nashville, Tenn., tells Fortune. “Now we’re having other winter viruses—including norovirus, a dominantly intestinal virus—also spreading this season.”

Cases of norovirus are rising in various areas of the U.S. and world, including Europe and Canada, and the pathogen is making headlines. It recently sickened nearly 500 on two U.S.-based cruise ships. And it’s once again responsible for myriad school closings, with one Detroit-area principal detailing a “rolling incidence of students throwing up” last week, causing the cancelation of classes from through Valentine’s Day.

This year’s norovirus season is particularly robust so far, experts tell Fortune. Here’s what you need to know to avoid the common wintertime menace known for sickening whole families—and schools, conferences, and cruise ships—at once.

What is norovirus and how does it spread?

Norovirus, often mistaken for the stomach flu, “spreads with remarkable ease,” Schaffner tells Fortune. Its nicknames include “winter vomiting disease” and “the cruise ship virus,” as it easily spreads among those in close quarters, he adds.

The illness usually moves from person to person via “fecal-oral” transmission. You can catch it by consuming contaminated food or water, and it’s the leading cause of foodborne illness in the U.S., according to the U.S. Centers for Disease Control and Prevention. You can also get it by touching a contaminated surface like a doorknob or light switch, then touching your mouth

It takes a very small amount of virus to get sick—so miniscule a microscope can’t always detect it, Dr. Ali Alhassani, head of clinical at Summer Health and a pediatrician at Boston’s Children’s Hospital, tells Fortune.

Because the virus is primarily passed through particles of feces invisible to the naked eye, it’s easy to unknowingly spread and contract the disease—if, for example, you don’t wash your hands well after using the restroom or changing a baby’s diaper. “It doesn’t take a lot to get people pretty sick,” Dr. Georges Benjamin, executive director of the American Public Health Association, tells Fortune. “That’s the main reason it’s so infectious.”

What’s more, if you’re near someone who is projectile vomiting, “you can actually be infected via aerosols,” Schaffner adds.

What are the symptoms of norovirus?

“In general, norovirus is very violent and inconvenient,” Dr. Luis Ostrosky, an infectious diseases specialist at University of Texas Health Houston, tells Fortune.

According to the CDC, common symptoms include:

• Diarrhea

• Vomiting

• Nausea

• Stomach pain

• Fever

• Headache

• Body aches

Symptoms usually occur within 12 to 48 hours of exposure, and last for one to three days. Because norovirus can cause repeated vomiting and diarrhea, “the biggest risk is getting very dehydrated,” Benjamin advises—especially among the young, elderly, and those with other medical conditions.

Why is norovirus circulating right now?

Norovirus is a common winter virus, though it’s also known to circulate via gatherings at other times of the year, like at spring or summer weddings or cruises.

The virus typically makes waves from November through March, Dr. Ali Alhassani, a pediatrician at Boston’s Children’s Hospital, tells Fortune.

“We are starting to see a little bit higher activity than usual, and a little bit on the early side, too,” Alhassani says, adding that the virus is on the uptick and perhaps approaching a peak in the U.S.

Alhassani is the head of clinical at Summer Health, a subscription-based pediatrics service accessible via text message in all 50 states. The service has seen a 13% increase in visits for “stomach bug” symptoms since December, he notes.

The current rise in cases could be linked to a recent U.S. outbreak of norovirus linked to raw oysters, Ostrosky says. While affected oysters were recalled nationally, it may still be driving cases.

We’ll all be experiencing infectious diseases more frequently, now that pandemic restrictions have been universally lifted, experts caution—at least for the near future. “Remember, we’re basically going from almost no cases of anything [during COVID lockdowns] to a bunch of cases of something,” be it RSV or flu earlier this winter, or norovirus now, Benjamin advises.

“We’re out and about sharing germs with each other again.”

February is a typical time for norovirus to take off, Schaffner adds, and “it’s really taking advantage of our having gotten together for the first time in several years.”

How can you best protect yourself and your family from norovirus?

The best advice, experts tell Fortune: Wash your hands frequently and stay away from others who are sick.

Dr. Alice Pong, clinical medical director of infectious diseases at Rady Children’s Hospital-San Diego, advises adults to be extra diligent about washing their hands before they eat—and to have their kids do the same. Alcohol-based hand sanitizers don’t work well on some viruses, including norovirus. So ditch the hand sanitizer in favor of actually washing your hands, she advises.

Alhassani recommends choosing household cleaners that promise to kill 99.9% of viruses. Such labeling informs consumers that products kill norovirus, a notoriously difficult task, he advises.

If you’re sick, be sure to stay home and avoid serving and preparing food for others, Ostrosky says. He emphasizes the importance of paid sick leave, particularly for food workers, in the case of a pathogen like norovirus.

There is not yet an approved vaccine for norovirus, though scientists are working on it, according to Schaffner. Thankfully, for most, “this is an illness that makes you miserable for two to three days, but then you recover,” he says.

“This is certainly a very vigorous norovirus year, and we’ll have to see how long it lasts, how quote on quote bad, or severe, it is,” he says. “It’s out there abundantly.”