Weekly test positivity rates have so far surged by 340% during the 2024–25 season, according to a Fortune analysis of Centers for Disease Control and Prevention (CDC) data recorded as three-week moving averages. Norovirus test positivity reached a season high of 22% the week ended Dec. 21, up from 5% the week ended Aug. 3. Just since the week ended Nov. 16, the rate had jumped from 11%, a 109% increase in five weeks.

These data may be an underestimation of the gastrointestinal disease’s reach. Norovirus can only be confirmed by laboratory PCR (polymerase chain reaction) testing and, especially during the holiday season, people who were ill may have been too sick or otherwise unable to access clinical care. Not to mention, CDC statistics available as of Dec. 27 don’t reflect disease spread over Christmas, Hanukkah, and Kwanzaa.

The CDC reported 91 norovirus outbreaks the week of Dec. 5, up from 69 the week before. By comparison, up to 41 outbreaks were reported during that same week the previous three seasons. The most norovirus outbreaks to occur that week between the 2012–13 and 2019–20 seasons was 65. However, these outbreaks only include data provided by the 14 NoroSTAT network states: Alabama, Colorado, Massachusetts, Michigan, Minnesota, Nebraska, New Mexico, North Carolina, Ohio, Oregon, South Carolina, Tennessee, Virginia, and Wisconsin.

Anyone can catch norovirus—the leading cause of vomiting, diarrhea, and foodborne illness in the country, per the CDC—at any time of year. But most outbreaks happen from November through April. In addition, we’re in the middle of respiratory virus season and holiday travel and gatherings. Unofficially, norovirus is one of the illnesses spurring a nationwide “quad-demic,” along with flu, COVID, and respiratory syncytial virus (RSV).

Even so, the current norovirus circulation isn’t unexpected. Roughly 2,500 norovirus outbreaks are reported in the U.S. each year, according to the CDC. Annually, it also accounts for:

• $2 billion in lost productivity and health care expenses
• 19–21 million illnesses
• 2.27 million outpatient clinic visits, mostly in young children
• 465,000 emergency department visits, mostly in young children
• 109,000 hospitalizations
• 900 deaths, mostly among adults 65+

What are the norovirus symptoms?

Though norovirus is sometimes called the “stomach bug” or “stomach flu” and may cause symptoms similar to those of influenza viruses, the contagious pathogen has nothing to do with the flu, the CDC notes.

Norovirus triggers gastrointestinal inflammation, which typically results in these symptoms:

• Body aches
• Diarrhea
• Fever
• Headache
• Nausea
• Stomach pain
• Vomiting

Even after you feel better, you can spread the illness to others for more than two weeks.

Is there a norovirus vaccine?

Not yet, which means practicing tried-and-true public health protocols are your best bet for avoiding infection, says Dr. Robert Hopkins Jr., medical director of the National Foundation for Infectious Diseases.

“It’s important to cover your coughs, ideally with your sleeve or with a handkerchief, not your hands,” Hopkins told Fortune just before Christmas. “Stay away from people who are sick, stay home when you’re ill, contact your health care professional about whether you need to get tested and about symptomatic measures.

“It’s important to stay active, particularly outdoor activities. Stay healthy as far as your eating and your fluid intake. And depending on what your health status is, you may really want to be careful about being in crowds, because that’s a natural breeding ground for those viruses to spread from person to person.”

And because alcohol-based hand sanitizers don’t effectively kill norovirus, don’t neglect your hand hygiene, says Dr. William Schaffner, a professor in the division of infectious diseases at Vanderbilt University Medical Center.

“Make ample use of soap and water,” Schaffner told Fortune in December. “It’s highly contagious and you can probably pick it up on the environment, on your fingers, and then when you touch your nose and mouth, that’s how the transmission occurs.”

The CDC also offers these tips for norovirus prevention:

• Clean and disinfect contaminated surfaces
• Cook shellfish thoroughly and wash fruits and vegetables
• Stay home when sick for 48 hours after symptoms stop
• Wash laundry in hot water

Where is norovirus spreading the most?

As of the week ended Dec. 21, norovirus test positivity was highest in the Midwest Census Region, which stretches from Ohio to the Dakotas, and as far south as Kansas and Missouri. The rate exceeded 25%, and marked the only one of the four census regions to have a test positivity rate higher than that of the nation.

The Northeast, which includes New England, New York, New Jersey, and Pennsylvania, had by far the lowest test positivity that week, 12%. This region was also the only one to show a decrease in test positivity since the week ended Dec. 14.

Norovirus outbreaks haven’t been limited to land. The CDC’s Vessel Sanitation Program reported five cruise ship outbreaks in December alone. Most recently, Cunard’s Queen Mary 2 reported back-to-back outbreaks on voyages beginning Dec. 14 and 21.

For more on norovirus:

• Moderna says it’s ‘one step closer’ to a norovirus vaccine as the virus spreads across the U.S.
• Public health experts are warning of a ‘quad-demic’ this winter. Here’s where flu, COVID, RSV, and norovirus are spreading
• Norovirus causes up to 21 million illnesses in the U.S. every year. Here’s what to know about the contagious disease that hand sanitizer can’t kill
• Norovirus is surging in the U.S. What you need to know, from length of symptoms to protecting yourself