Stomach flu is a viral infection of the stomach and intestines that causes vomiting, diarrhea, and abdominal pain. Its medical name is viral gastroenteritis, and despite the nickname, it has nothing to do with influenza. Most cases resolve on their own within one to three days, but the illness can cause significant fluid loss that requires careful attention, especially in young children and older adults.
How It Differs From the Actual Flu
The confusion is understandable, but stomach flu and influenza are completely different illnesses caused by different viruses attacking different parts of your body. Influenza targets the respiratory system: your nose, throat, and lungs. Stomach flu targets the gastrointestinal tract: your stomach and intestines. Influenza gives you a cough, congestion, and body aches. Stomach flu gives you vomiting and diarrhea. The two can overlap with symptoms like fever and fatigue, but their core effects are distinct.
What Causes It
Four main virus families are responsible for most cases of viral gastroenteritis: rotavirus, norovirus, adenovirus, and astrovirus. Which one you’re most likely to encounter depends largely on your age.
In children, rotavirus is the dominant cause, responsible for 29% to 45% of gastroenteritis hospitalizations worldwide. Norovirus ranks second in children, with the highest infection rates in kids under two years old. Adenovirus and astrovirus account for a smaller share, roughly 5% and 2% of hospitalized cases respectively.
In adults, norovirus is the primary culprit. It’s the virus behind most of the outbreaks you hear about on cruise ships, in restaurants, and in nursing homes. It spreads efficiently through contaminated food, water, and surfaces, and it takes very few viral particles to cause infection.
What Happens Inside Your Body
When one of these viruses reaches your intestines, it invades the cells lining the intestinal wall and uses them to replicate. This hijacking disrupts the cells’ normal function. Your intestinal lining produces enzymes that help break down and absorb food, and once the virus interferes with that process, nutrients and water that would normally be absorbed instead stay in the intestine. That’s what causes the watery diarrhea.
On top of that, the virus releases toxins that directly damage and destroy intestinal cells, which triggers additional fluid loss into the intestine. Your body responds with inflammation, nausea, cramping, and vomiting as it tries to expel the invader. The good news: your intestinal lining regenerates quickly, which is why most people recover within days.
Symptoms and Timeline
Symptoms typically appear one to three days after exposure, depending on the virus. Norovirus has a slightly faster onset, usually showing up within one to two days. Rotavirus takes one to three days. In both cases, the illness often arrives suddenly. You might feel fine in the morning and be vomiting by the afternoon.
The core symptoms include:
- Watery diarrhea (usually not bloody)
- Nausea and vomiting
- Abdominal cramps and pain
- Low-grade fever
- Fatigue and muscle aches
Most people with norovirus start feeling better within one to two days after symptoms begin. Rotavirus can last a bit longer, particularly in young children. The vomiting usually subsides first, followed by gradual improvement in diarrhea over the next day or two.
How Long You’re Contagious
This is where stomach flu gets tricky. You’re contagious before you even know you’re sick, and you stay contagious well after you feel better. People with rotavirus can spread the virus before symptoms start and for up to two weeks after recovery. With norovirus, you remain contagious for several days after symptoms resolve, and the virus can persist in stool for two weeks or more.
That extended window is one reason stomach flu spreads so easily through households, daycare centers, and shared living spaces. Someone who feels perfectly fine can still be shedding virus.
Dehydration: The Main Risk
The virus itself isn’t usually dangerous. Dehydration is. When you’re losing fluid from both ends simultaneously, your body can fall behind quickly. Children, older adults, and people with weakened immune systems are most vulnerable.
Early signs of dehydration include dry mouth, decreased urination, dark-colored urine, and feeling lightheaded when you stand up. In children, watch for fewer wet diapers, crying without tears, and unusual sleepiness or fussiness. More serious dehydration shows up as sunken eyes, skin that stays pinched when you pull it (rather than snapping back), a weak or rapid pulse, and a generally limp or unresponsive appearance. Research on dehydration in children with gastroenteritis found that when two or more of those signs appear together, it typically indicates a fluid loss of at least 5% of body weight, which needs prompt medical attention. Untreated severe dehydration can lead to dangerous electrolyte imbalances and kidney problems.
How to Manage It at Home
There’s no antiviral medication that treats stomach flu. Recovery is about managing symptoms and replacing lost fluids until your body clears the virus on its own.
Oral rehydration is the cornerstone of treatment. Water alone isn’t ideal because you’re also losing electrolytes like sodium and potassium through vomiting and diarrhea. Oral rehydration solutions, available at most pharmacies, contain a balanced mix of water, salts, and a small amount of sugar (typically 2% to 3% carbohydrate) designed to maximize absorption. The sugar isn’t just for taste; it helps your intestine pull sodium and water back into your body through a transport mechanism the virus doesn’t fully disable.
If you can’t keep liquids down, take small, frequent sips rather than gulping large amounts. A few tablespoons every five to ten minutes often stays down better than a full glass. For children, the same approach applies with even smaller amounts. Popsicles or ice chips can help if sipping is difficult.
Once the vomiting eases, you can reintroduce bland, easy-to-digest foods. There’s no need to follow a strict diet, but most people feel better starting with simple carbohydrates like toast, rice, or crackers before moving back to their normal meals. Avoid dairy, caffeine, alcohol, and fatty or heavily spiced foods until your gut has had a chance to recover.
Preventing the Spread
Norovirus is remarkably hardy. It survives on countertops, doorknobs, and other hard surfaces long enough to infect the next person who touches them. Standard household cleaners often aren’t enough to kill it. The CDC recommends using a bleach solution of 5 to 25 tablespoons of standard household bleach per gallon of water to disinfect contaminated surfaces.
Handwashing with soap and water is more effective than alcohol-based hand sanitizers against norovirus. Wash thoroughly after using the bathroom, before preparing food, and after caring for someone who’s sick. If someone in your household has stomach flu, clean any contaminated surfaces, bedding, and clothing promptly, and try to avoid sharing towels and utensils during the contagious period.
The Rotavirus Vaccine
While there’s no vaccine for norovirus, rotavirus vaccination has dramatically reduced the burden of stomach flu in young children. The vaccine, given orally in two or three doses starting at two months of age, provides 85% to 98% protection against severe rotavirus illness and hospitalization during an infant’s first year, and 74% to 87% protection against rotavirus illness of any severity. All doses should be completed before a child turns eight months old, with the first dose given before 15 weeks of age. Since the vaccine’s introduction, rotavirus hospitalizations in children have dropped substantially in countries with routine vaccination programs.