You get the stomach flu by swallowing a virus, most often norovirus, that has made its way from an infected person’s stool or vomit into your mouth. This can happen through contaminated food, contaminated water, touching a dirty surface and then touching your face, or close contact with someone who is sick. The amount of virus needed to make you sick is remarkably small: as few as 10 to 100 viral particles can trigger a full infection, while a single sick person sheds millions of them in every bout of diarrhea.
The Viruses Behind It
The stomach flu isn’t actually the flu. Influenza attacks your lungs; gastroenteritis attacks your intestines. Two viruses cause the vast majority of cases. Norovirus is the most common cause of foodborne illness worldwide and hits both children and adults. Rotavirus is the leading cause of stomach flu in young children, particularly infants and toddlers, though widespread vaccination has reduced its impact in many countries.
A newer strain of norovirus, called GII.17, has been surging in the United States. During the 2024-25 season, it accounted for over 75% of outbreaks, replacing the strain that had dominated for years. The season also started unusually early, in October rather than December, and reported outbreaks nearly doubled compared to the same period the previous year.
How the Virus Spreads
The primary route is fecal-oral transmission. That sounds unpleasant because it is: microscopic traces of an infected person’s stool or vomit end up being swallowed by someone else. In practice, this happens through several everyday scenarios.
- Contaminated food. An infected food worker touches ready-to-eat items like salads or fruit with bare hands. This is the single most common source, linked to roughly 39% of reported norovirus outbreaks. Leafy greens, fresh fruits, and shellfish (especially oysters harvested from contaminated water) are the foods most frequently involved.
- Person-to-person contact. Caring for a sick family member, sharing utensils, or shaking hands with someone who didn’t wash up after using the bathroom can all transfer the virus.
- Contaminated surfaces. Norovirus can survive on hard surfaces like countertops, doorknobs, and plastic for more than two weeks. You touch the surface, then touch your mouth, and the virus is in.
- Airborne droplets. When someone vomits, the virus easily becomes airborne. Standing nearby in a confined space, like a cruise ship cabin or a shared bathroom, can be enough exposure.
- Contaminated water. Less common but still relevant, particularly with well water or recreational water sources.
Confined spaces amplify every one of these routes. Cruise ships, dormitories, nursing homes, and daycare centers are classic hotspots because people share surfaces, food, and air in close quarters.
How Quickly Symptoms Start
Norovirus has one of the shortest incubation periods of any common infection. Symptoms typically appear 12 to 48 hours after exposure. You can go from feeling perfectly fine at breakfast to vomiting by dinner the same day.
The illness itself usually lasts one to two days, though some people feel sick for up to two weeks. The hallmark symptoms are watery diarrhea, vomiting, nausea, and stomach cramps. Some people also develop a low-grade fever, headache, or body aches. One important distinction: the diarrhea is usually not bloody. Bloody diarrhea points to a different, more serious type of infection that warrants prompt medical attention.
How Long You Stay Contagious
Here’s the part most people underestimate. You are contagious before your symptoms start and remain contagious for days after you feel better. The virus continues to shed in your stool even once the vomiting and diarrhea have stopped. This is why outbreaks spread so efficiently through households and workplaces: people return to their routines feeling recovered but are still passing the virus along.
Given that a sick person sheds millions of viral particles in their stool and it only takes somewhere between 10 and 100 particles to infect someone new, even tiny lapses in hand hygiene can keep the chain of transmission going.
Why Hand Sanitizer Isn’t Enough
Alcohol-based hand sanitizers do not work well against norovirus. The virus lacks the outer coating that alcohol is designed to break down, so a squirt of sanitizer gives you a false sense of protection. Soap and water is the only reliable way to remove the virus from your hands. You can use hand sanitizer as an extra step, but it should never replace actual handwashing.
This matters especially in settings where hand sanitizer stations are the default, like hospital lobbies, office buildings, and school entrances. If norovirus is circulating, those dispensers are not protecting you the way you might assume.
Protecting Yourself at Home
If someone in your household has the stomach flu, the virus is almost certainly on surfaces throughout your home. Clean any areas the sick person has touched, particularly bathrooms, with a bleach-based disinfectant. Standard household cleaners may not be strong enough to kill norovirus.
Wash contaminated clothing and bedding promptly, using the hottest water setting available, and dry on high heat. Avoid preparing food for others while you’re sick and for at least two days after your symptoms resolve. The foods most likely to carry the virus forward are anything served raw or handled after cooking, so this period of caution matters.
Dehydration is the biggest practical risk during an active infection, especially for young children and older adults. Frequent vomiting and diarrhea drain fluids fast. Warning signs of dehydration include decreased urination, a dry mouth and throat, dizziness when standing, and in children, crying with few or no tears or unusual sleepiness.
Why Some People Get It Repeatedly
Unlike many viral infections, a bout of norovirus does not give you lasting immunity. Multiple strains circulate each season, and the virus mutates frequently. The current dominance of the GII.17 strain in the U.S. means that even people who caught a different strain recently have little to no protection. This is also why there is no widely available vaccine for norovirus yet, though rotavirus vaccines for infants have been highly effective at reducing that specific virus.
Your best ongoing defense is consistent handwashing with soap and water, careful food handling, and staying away from others when you’re actively sick. In a household outbreak, isolating the sick person’s bathroom, if possible, and disinfecting shared surfaces multiple times a day can significantly reduce the chance that everyone in the home ends up ill.