The stomach flu, known medically as viral gastroenteritis, causes watery diarrhea, nausea, vomiting, stomach cramps, and a low-grade fever. Symptoms typically appear 24 to 48 hours after exposure to the virus and last anywhere from three to eight days, though some cases stretch to two weeks. Despite the name, it has nothing to do with influenza. The virus targets your intestines, not your respiratory system.
Core Symptoms
The hallmark of stomach flu is watery, nonbloody diarrhea. Most people also experience nausea, vomiting, or both, along with stomach cramps and abdominal pain. These gut symptoms tend to hit in waves, with periods of relative calm between bouts. The diarrhea is typically frequent and urgent, sometimes making it difficult to leave the bathroom for long stretches.
Beyond the digestive symptoms, you may notice muscle aches, headache, fatigue, and a general feeling of being unwell (sometimes described as malaise). A low-grade fever is common but not universal. Loss of appetite is almost guaranteed, and even the thought of food can trigger nausea in the early days. The combination of vomiting and diarrhea is what makes stomach flu particularly draining, because your body is losing fluids from both directions at once.
How Quickly Symptoms Appear
The incubation period depends on which virus you’ve caught. Norovirus, the most common cause in adults, typically takes 24 to 48 hours to produce symptoms after exposure. Rotavirus, which more commonly affects young children, has a similar incubation period of about two days. This delay is one of the key differences between stomach flu and food poisoning. Food poisoning tends to strike fast, usually within two to six hours of eating contaminated food, while stomach flu builds more gradually.
Once symptoms start, expect the worst of it during the first one to three days. Vomiting often eases before diarrhea does. Most people feel significantly better within a week, but loose stools and low energy can linger for a few days after the acute phase passes.
Stomach Flu vs. Food Poisoning
Because the symptoms overlap so heavily, it can be hard to tell these two apart. Both cause diarrhea and vomiting. Both can cause fever. A few patterns help distinguish them.
- Speed of onset: Food poisoning hits within hours of eating something bad. Stomach flu takes a day or two to develop.
- Fever and chills: Systemic effects like fever, chills, and body aches are more common with viral gastroenteritis than with food poisoning, though neither is a hard rule.
- Bloody diarrhea: If your diarrhea contains blood, that points away from typical stomach flu and toward a bacterial infection or food poisoning. The rapid onset of severe diarrhea can disrupt the intestinal lining enough to cause bleeding. Bloody stool generally signals a more serious infection that needs medical attention.
Dehydration: The Biggest Risk
The symptoms themselves are miserable but usually not dangerous. The real threat is dehydration, which develops when you’re losing more fluid through vomiting and diarrhea than you can replace by drinking. This is especially dangerous for infants, young children, and older adults.
In adults, signs of dehydration include urinating less than usual, dark-colored urine, dry mouth, dizziness when standing, and skin that doesn’t spring back quickly when you pinch it on the back of your hand. In babies and toddlers, watch for fewer wet diapers (none for three hours is a red flag), a dry mouth, crying without tears, and unusual sleepiness or irritability. Skin that stays “tented” after a gentle pinch instead of flattening right away is another sign in both children and adults.
Sipping small amounts of fluid frequently works better than trying to drink a full glass at once, which can trigger more vomiting. Oral rehydration solutions are ideal because they replace both water and the electrolytes your body is losing. For mild cases, broth, diluted juice, and water can be enough to stay ahead of the fluid loss.
Symptoms in Babies and Young Children
Young children often can’t tell you what they’re feeling, so you have to read the signs. Frequent vomiting is the most obvious one, and it can come on suddenly. Watery diarrhea, fussiness, refusal to eat or drink, and lethargy all point to stomach flu. Babies may pull their legs toward their belly during cramps.
Children dehydrate faster than adults because of their smaller body size and higher metabolic rate. A baby who has no wet diapers for three hours, has a dry mouth, or seems unusually drowsy needs prompt medical evaluation. Frequent vomiting in an infant also warrants a call to the pediatrician, even before dehydration sets in, because the window between “uncomfortable” and “dangerously dry” is shorter in small bodies.
When Symptoms Point to Something Else
Most stomach flu resolves on its own within a week. Certain symptoms, however, suggest something beyond a routine viral infection. Vomiting or diarrhea lasting more than two days in an adult, inability to keep any liquids down for 24 hours, blood in your stool, or a high fever (not the mild, low-grade type typical of stomach flu) all warrant medical attention.
Diagnosis is almost always based on symptoms alone. Lab tests exist but are rarely needed for a straightforward case. If your symptoms are unusually severe, prolonged, or include bloody diarrhea, a provider may test for bacterial causes or other conditions that mimic stomach flu, such as food poisoning, parasitic infections, or inflammatory bowel disease flares.