A stomach bug, known medically as viral gastroenteritis, is an intestinal infection that causes watery diarrhea, vomiting, nausea, stomach cramps, and sometimes fever. It’s one of the most common illnesses worldwide, triggered by a handful of highly contagious viruses that spread quickly through households, schools, and anywhere people are in close contact. Most cases resolve on their own within one to three days, but the main risk, especially for young children and older adults, is dehydration.
Which Viruses Cause It
Despite being called the “stomach flu,” a stomach bug has nothing to do with influenza. The real culprits are a small group of viruses that target the lining of your intestines. Norovirus is the most common cause in both children and adults and is the leading source of foodborne illness globally. Rotavirus is the most common cause in young children worldwide, particularly infants who pick it up by putting contaminated fingers or objects in their mouths.
A few less well-known viruses, including adenovirus, astrovirus, and sapovirus, account for a smaller share of cases. The symptoms are broadly similar regardless of which virus is responsible, though the severity and exact timeline can vary slightly.
How It Spreads
Stomach bugs spread primarily through the fecal-oral route, which sounds alarming but is simpler than it seems. Microscopic particles of stool or vomit from an infected person end up on hands, surfaces, food, or water, and then make their way into someone else’s mouth. This can happen when an infected person touches a doorknob or kitchen counter, when contaminated food is shared, or even when tiny droplets from vomiting land on nearby surfaces or become airborne and are inhaled.
This is why outbreaks tear through families so efficiently. A single episode of vomiting can contaminate a bathroom, and the virus can survive on hard surfaces for days. Norovirus is notoriously difficult to kill: alcohol-based hand sanitizer does not work well against it. Soap and water is the only reliable way to wash the virus off your hands, and the CDC recommends using hand sanitizer only as a supplement, never a substitute, for handwashing during an outbreak.
Symptoms and Timeline
Symptoms typically appear 12 to 48 hours after exposure to norovirus. The illness usually starts suddenly with nausea, followed by vomiting and watery diarrhea. The clinical definition is three or more loose or watery stools in a single day, often accompanied by stomach cramps and low-grade fever. Some people experience all of these symptoms at once, while others may only have diarrhea or only vomiting.
Most people feel significantly better within one to three days. The worst of the vomiting often passes within the first 24 hours, while diarrhea can linger a bit longer. You remain contagious while you have symptoms and for at least a couple of days after they stop, which is why it’s important to stay home and keep washing your hands thoroughly even after you start feeling better.
The Real Danger: Dehydration
The virus itself isn’t usually dangerous. The real risk is losing too much fluid through vomiting and diarrhea faster than you can replace it. Young children, older adults, and people with weakened immune systems are most vulnerable.
In children, the best early warning signs of significant dehydration (more than 5% of body weight lost as fluid) are no tears when crying, sunken eyes, and skin that stays tented when you gently pinch it on the abdomen or thigh. A child who is restless, unusually irritable, or intensely thirsty is showing signs of moderate dehydration. Severe dehydration looks more alarming: extreme sleepiness, refusal to drink, cool or bluish hands and feet, rapid breathing, and a weak, fast pulse. In infants, a sunken soft spot on the head is another red flag.
In adults, watch for dark urine or very little urine output, dizziness when standing, dry mouth, and a feeling of lightheadedness that doesn’t improve with rest.
How to Stay Hydrated
Replacing lost fluids is the single most important thing you can do. For children, oral rehydration solutions (sold at pharmacies under brand names like Pedialyte) are the gold standard because they contain the right balance of sugar and electrolytes to help the intestines absorb water efficiently. If your child is vomiting frequently, start small: offer a teaspoon (about 5 mL) at a time using a syringe or medicine dropper, then gradually increase the amount as they keep it down. For each episode of vomiting or watery stool, children under about 22 pounds need roughly 2 to 4 ounces of rehydration solution, and larger children need 4 to 8 ounces.
Breastfeeding should continue throughout the illness, even during the worst of the vomiting. Breast milk is well tolerated and provides both fluid and calories.
Adults can use oral rehydration solutions too, or sip water and clear broths. The key is frequency: small, steady amounts are far easier on an irritated stomach than gulping a full glass at once. Sports drinks are better than nothing but contain more sugar and less sodium than ideal rehydration formulas.
What to Eat During Recovery
The old advice to stick to the BRAT diet (bananas, rice, applesauce, toast) has fallen out of favor. While those foods are gentle on the stomach, a strict BRAT diet lacks calcium, protein, vitamin B12, and fiber, and following it for more than about 24 hours can actually slow recovery by depriving your body of the nutrients it needs to heal the intestinal lining.
Current guidance is simpler: eat as tolerated. When you feel ready, start with soft, bland foods, but move back toward your normal diet as soon as you can manage it. For children, the American Academy of Pediatrics specifically advises against restricting kids to BRAT foods because the nutritional gaps are even more problematic in small, growing bodies. A day of crackers and bananas at your sickest is fine, but push toward real meals once the worst has passed.
Preventing a Stomach Bug
Thorough handwashing with soap and water is your best defense, especially after using the bathroom, changing diapers, and before preparing or eating food. Wash for at least 20 seconds, and remember that hand sanitizer alone is not effective against norovirus.
If someone in your household is sick, clean contaminated surfaces with a bleach-based cleaner rather than standard household sprays. Wash soiled clothing and bedding on the hottest cycle the fabric allows, and try to isolate the sick person’s bathroom if possible. Avoid preparing food for others while you’re ill and for at least two days after symptoms stop.
For rotavirus specifically, a vaccine is available for infants and is part of the routine childhood immunization schedule in many countries. It has dramatically reduced hospitalizations from rotavirus in young children. There is no vaccine for norovirus.