The agency reports national norovirus trends as three-week moving averages of positive test rates. So far in the 2024–25 season, infections peaked the week of Nov. 30, 2024, at 17.06% positivity.

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, commonly called the stomach flu, though it’s unrelated to influenza, “spreads with remarkable ease,” Dr. William Schaffner, professor of infectious diseases at Vanderbilt University Medical Center in Nashville, 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 CDC. You can also get it by touching a contaminated surface like a doorknob or light switch and 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 subscription-based pediatrics service Summer Health and a pediatrician at Boston 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 the University of Texas Health Science Center at Houston, tells Fortune.

According to the CDC, common symptoms include:

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

Symptoms usually occur within 12–48 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, the elderly, and those with other medical conditions.

Is there a treatment for norovirus?

Unfortunately, there is no specific treatment for the illness. Antibiotic drugs, which fight bacteria, can’t treat norovirus.

You should be sure, however, to keep hydrated, to replace fluids lost from vomiting and diarrhea. If you’re caring for a child with norovirus, watch for signs of dehydration, including crying few or no tears and being unusually sleepy or fussy. If you think you’re severely dehydrated or that someone you’re caring for is, call your health care provider, the CDC advises.

While sports drinks and other caffeine-free and nonalcoholic drinks may help treat mild dehydration, the CDC stresses that those fluids might not replace nutrients and minerals. The agency recommends over-the-counter rehydration fluids, such as Pedialyte.

Is there a vaccine for norovirus?

There is still no vaccine for norovirus, however there is hope that there will be one soon.

Earlier this year, Moderna began a clinical trial for a norovirus vaccine. Like its COVID-19 vaccine, the shot for norovirus would be a messenger RNA, or mRNA, vaccine. Moderna says the aim of these vaccines is to teach the body how to make a specific protein to help your immune system fight certain diseases. The vaccine is supposed to train your immune system cells to recognize these proteins and protect against the strain of infection if you’re exposed. 

It’s been difficult to create an effective vaccine for norovirus so far, because of how much the virus mutates and shifts. Experts told the BBC they’re optimistic, since this vaccine contains three of the most common strains of norovirus.

The trial is taking place from September 2024 to May 2027. 

When is norovirus season?

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, Alhassani tells Fortune.

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, flu, or norovirus, Benjamin advises.

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

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

Nuttawan Jayawan—Getty Images

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

The best advice, experts tell Fortune: 

• Wash your hands frequently with soap and water 
• Clean and disinfect surfaces with antimicrobial products (the CDC also recommends a chlorine bleach solution)
• Stay away from others who are sick
• Don’t serve or prepare food for others if you’re 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. 

And don’t forget the laundry: Soiled clothes and linens should be washed with detergent and hot water for the maximum available cycle, the CDC says. Machine-dry them at the hottest setting.

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 cautions, emphasizing 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.

For more on norovirus:

• Norovirus rates have skyrocketed by 340% this season. Here’s where the ‘winter vomiting disease’ is spreading and why
• 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 is surging in the U.S. What you need to know, from length of symptoms to protecting yourself

Additional reporting by Ani Freedman and Lindsey Leake.