Stomach flu is a viral infection of the intestines that causes nausea, vomiting, diarrhea, and stomach cramps. Its medical name is viral gastroenteritis, and despite the nickname, it has nothing to do with influenza. Norovirus and rotavirus are the two most common culprits, and together they account for the vast majority of cases. Worldwide, norovirus alone causes roughly 685 million cases of acute gastroenteritis every year.
What Causes It
Several viruses can trigger gastroenteritis, but norovirus dominates in adults and older children. It’s extraordinarily contagious. You catch it by swallowing microscopic particles of feces or vomit from an infected person, which sounds dramatic but happens easily: a contaminated doorknob, a shared towel, food prepared by someone who’s sick, or even tiny airborne droplets released when a nearby person vomits.
Rotavirus is the leading cause in young children. Before vaccination became widespread, nearly every child contracted rotavirus by age five. Other viruses, including adenovirus and astrovirus, cause a smaller share of cases.
How It Spreads
Norovirus travels through contaminated food, water, and surfaces. A sick person touches a countertop, you touch the same spot, then touch your mouth. Food handlers who are ill can contaminate entire buffets or restaurant kitchens. Shellfish, particularly oysters, can harbor the virus when harvested from contaminated water. Produce irrigated with tainted water is another route.
Contaminated drinking or recreational water spreads it too, especially when treatment systems fail or septic tanks leak into wells. The virus thrives in close quarters, which is why outbreaks tear through cruise ships, dormitories, nursing homes, and daycare centers so quickly.
Symptoms and Timeline
After exposure, symptoms typically appear within 12 to 48 hours. The hallmark signs are watery diarrhea, vomiting, nausea, and abdominal cramps. Low-grade fever, headaches, and body aches are common but not universal. Most people start feeling better within one to three days, though fatigue and mild digestive sensitivity can linger a few days longer.
Children and older adults tend to get hit harder. Young children may vomit more frequently, while older adults are more vulnerable to the dehydration that comes with sustained fluid loss. Healthy adults usually recover without complications, but dehydration is the main risk for everyone.
Stomach Flu vs. Food Poisoning
The two conditions feel similar but behave differently. Food poisoning, caused by bacterial toxins already present in spoiled food, hits fast. Symptoms usually begin within two to six hours of eating the contaminated meal. Stomach flu has a longer incubation period of 24 to 48 hours, so you often can’t pinpoint exactly when or where you picked it up.
Duration differs too. Food poisoning tends to be brief, often resolving within a day. Viral gastroenteritis generally lingers for about two days and sometimes longer. If multiple people who ate the same meal all get sick within hours, food poisoning is the more likely explanation. If family members fall ill one by one over several days, a virus is probably making the rounds.
Staying Hydrated During Recovery
There’s no antiviral medication for stomach flu. Recovery comes down to managing symptoms and replacing lost fluids. Dehydration is the biggest danger, especially for children, older adults, and anyone who can’t keep liquids down.
Oral rehydration solutions are the gold standard. Both the CDC and the American Academy of Pediatrics recommend commercially available low-osmolarity formulas for adults and children with acute gastroenteritis. These contain a precise balance of glucose and electrolytes that helps your intestines absorb water more efficiently than plain water alone. For mild cases, clear broths, diluted juices, and electrolyte drinks can also help. Avoid sugary sodas and undiluted fruit juice, which can worsen diarrhea.
Small, frequent sips work better than gulping large amounts, especially if vomiting is still active. Once you can tolerate liquids, bland foods like toast, rice, bananas, and plain crackers are gentle starting points. Most people can return to a normal diet within a day or two of symptom improvement.
When Symptoms Are More Serious
Most stomach flu cases resolve on their own, but certain warning signs suggest dehydration or complications that need medical attention. Watch for diarrhea lasting more than 24 hours, inability to keep any fluids down, bloody or black stool, fever at or above 102°F, and unusual confusion or sleepiness. In infants and small children, fewer wet diapers, a dry mouth, and crying without tears are key red flags. These situations call for professional evaluation, as intravenous fluids may be needed to correct dehydration.
How Long You’re Contagious
You’re most contagious while actively symptomatic and for at least a few days after symptoms stop. Viral particles continue to shed in stool even after you feel fine, which is why handwashing remains critical during recovery. Returning to work or school while still experiencing diarrhea or vomiting puts others at significant risk. A good rule of thumb is to wait at least 48 hours after your last episode of vomiting or diarrhea before resuming normal activities.
Prevention
Thorough handwashing with soap and water is the single most effective defense. Alcohol-based hand sanitizers are less reliable against norovirus specifically, so actual soap and running water matters here. Wash your hands after using the bathroom, before eating, and before preparing food.
For rotavirus, vaccination has been a game changer. After the rotavirus vaccine was introduced, hospitalization rates for young children dropped dramatically. Children aged 6 to 23 months saw the greatest benefit, with hospitalizations falling by 50%. The vaccine is given in infancy as part of the routine immunization schedule. No vaccine currently exists for norovirus, though several are in development.
If someone in your household is sick, clean contaminated surfaces with a bleach-based disinfectant. Wash soiled clothing and bedding promptly using hot water. Keep the sick person’s utensils, towels, and drinking glasses separate from everyone else’s until they’ve been symptom-free for at least two days.