A stomach virus, known medically as viral gastroenteritis, is an infection of the intestines caused by one of several highly contagious viruses. It triggers nausea, vomiting, watery diarrhea, and abdominal pain that typically last one to three days before clearing on their own. Despite being commonly called the “stomach flu,” it has nothing to do with influenza, which is a respiratory illness.
Which Viruses Cause It
Four viruses are responsible for most cases of viral gastroenteritis: norovirus, rotavirus, adenovirus, and astrovirus. Norovirus is the leading cause in adults and older children, while rotavirus and adenovirus are more common in infants and toddlers, particularly those between six months and two years old. Rotavirus alone accounts for roughly 37% of gastroenteritis cases in children under three, though widespread vaccination has reduced its impact significantly in countries with routine childhood immunization programs.
Norovirus deserves special attention because of how remarkably contagious it is. As few as 18 viral particles can cause an infection, yet a single gram of stool from an infected person can contain billions of infectious doses. This explains why norovirus tears through households, cruise ships, schools, and nursing homes so quickly. A tiny amount of contamination on a doorknob, countertop, or shared food is more than enough to spread the illness.
How It Spreads
Stomach viruses spread through what’s called the fecal-oral route, which sounds unpleasant because it is. The virus leaves an infected person’s body through stool or vomit, then enters someone else’s mouth via contaminated hands, surfaces, food, or water. You can pick it up by touching a contaminated surface and then touching your face, eating food prepared by someone who’s infected, or caring for a sick person without thorough hand hygiene afterward.
One important detail many people don’t realize: you remain contagious well after you feel better. The CDC notes that people can continue shedding norovirus for two weeks or more after symptoms resolve. The highest risk of transmission is during active illness and the first 48 hours after symptoms stop, which is why the recommendation is to stay home for at least two full days after your last episode of vomiting or diarrhea.
Symptoms and Timeline
After exposure to norovirus, symptoms typically appear within 12 to 48 hours. The hallmark of viral gastroenteritis is three or more episodes of loose or watery diarrhea in a day, often accompanied by nausea, vomiting, stomach cramps, and a low-grade fever. Some people experience all of these at once, while others deal mostly with vomiting or mostly with diarrhea.
Most cases resolve within one to three days. If symptoms persist beyond a week, or if you develop high fever, bloody diarrhea, prolonged vomiting, or severe abdominal pain, something other than a routine stomach virus may be going on. Those symptoms can point to bacterial infections, appendicitis, or other conditions that need different treatment.
Stomach Virus vs. Food Poisoning
The symptoms of a stomach virus and food poisoning overlap almost completely, which is why people often use the terms interchangeably. Both cause nausea, vomiting, diarrhea, and cramping. The practical difference is the cause: a stomach virus is a viral infection that spreads person to person, while food poisoning is typically caused by bacterial toxins in contaminated food. Food poisoning tends to come on faster (sometimes within hours of eating) and often affects multiple people who shared the same meal. In either case, the body’s main need is the same: staying hydrated while the illness runs its course.
Why Dehydration Is the Real Danger
The stomach virus itself is rarely dangerous for otherwise healthy people. The risk comes from dehydration, especially in young children, older adults, and anyone with a weakened immune system. When you’re losing fluids rapidly through vomiting and diarrhea, your body can fall behind on water and electrolytes faster than you might expect.
In adults, signs of dehydration include extreme thirst, dark urine, urinating less than usual, dizziness, confusion, and fatigue. In infants and young children, watch for no wet diapers for three hours, a dry mouth, crying without tears, sunken eyes, a sunken soft spot on top of the head, rapid heart rate, and skin that stays pinched instead of bouncing back when you press it. Severe dehydration in any age group needs medical attention.
How to Stay Hydrated
Plain water helps, but it doesn’t replace the sodium and potassium your body is losing. Oral rehydration solutions are specifically designed for this. They contain a precise balance of sugar and salt that helps your intestines absorb water more efficiently. You can find these over the counter at any pharmacy.
Common household drinks like apple juice, cola, tea, and sports drinks are not good substitutes, particularly for children. Apple juice contains virtually no sodium and extremely high sugar (250 grams per liter compared to about 13.5 in a modern oral rehydration solution). Cola has only 3 milliequivalents of sodium per liter. These imbalanced fluids can actually worsen diarrhea by pulling more water into the intestines through osmosis. For adults who are mildly dehydrated, broth and diluted sports drinks can bridge the gap, but for children with significant fluid loss, a proper rehydration solution is the better choice.
Take small, frequent sips rather than large gulps, especially if you’re still vomiting. A few tablespoons every 10 to 15 minutes is easier on the stomach than drinking a full glass at once.
What to Eat During Recovery
You may have heard of the BRAT diet (bananas, rice, applesauce, toast) as the standard advice for a stomach bug. It’s no longer recommended as a strict regimen. The American Academy of Pediatrics considers it too restrictive for children, noting that it lacks protein, calcium, vitamin B12, and fiber. Following it for more than 24 hours can actually slow recovery by depriving your body of the nutrients it needs to heal.
The current approach is simpler: eat as tolerated. When you feel ready to try food, start with small portions of soft, bland options. Bananas and plain rice are fine, but so are scrambled eggs, skinless chicken or turkey, and cooked vegetables. Smaller meals tend to sit better than large ones. As your appetite returns, gradually expand what you eat. Your body needs fuel to recover, so don’t restrict yourself to crackers and toast longer than necessary.
Preventing the Spread
Handwashing with soap and water is the single most effective way to prevent stomach viruses. This is one area where alcohol-based hand sanitizer falls short. The CDC is clear that hand sanitizer does not work well against norovirus and should not be used as a substitute for washing your hands. You can use it as a supplement when soap and water aren’t available, but scrubbing with soap for at least 20 seconds is what actually removes the virus.
If someone in your household is sick, clean contaminated surfaces with a bleach-based disinfectant rather than standard all-purpose cleaners. Wash soiled clothing and bedding on the hottest setting and dry them thoroughly. Keep the sick person’s utensils, towels, and drinking glasses separate. And remember the 48-hour rule: the infected person should avoid preparing food for others or returning to work or school until two full days after their last symptoms.
For young children, the rotavirus vaccine has dramatically reduced hospitalizations and severe illness worldwide. It’s given as an oral dose during infancy and is part of the routine childhood immunization schedule in most countries. There is currently no vaccine available for norovirus.