A stomach bug typically causes watery diarrhea, nausea, vomiting, stomach cramps, and sometimes a low-grade fever. Symptoms usually appear 1 to 3 days after exposure and last anywhere from 1 to 3 days in most cases, though they can occasionally linger up to 14 days. Norovirus, the most common culprit, causes an estimated 19 to 21 million illnesses in the United States each year.
The Main Symptoms
The hallmark of a viral stomach bug is watery diarrhea that typically does not contain blood. Alongside diarrhea, most people experience nausea, vomiting (or both), and abdominal cramping or pain. A low-grade fever is common but not universal. Some people also notice muscle aches, headaches, or general fatigue.
These symptoms can range from mild to severe. You might have only a day of nausea and loose stools, or you might spend two to three days dealing with frequent vomiting and diarrhea that keeps you close to the bathroom. The severity often depends on which virus you picked up and how strong your immune system is at the time.
How Quickly Symptoms Start and How Long They Last
With norovirus, symptoms typically develop 12 to 48 hours after exposure. Other viruses follow a similar window, generally 1 to 3 days. The illness hits fast: you might feel perfectly fine in the morning and be vomiting by evening.
Most people recover within 1 to 3 days. Some infections, particularly those caused by adenoviruses, can produce diarrhea lasting 1 to 2 weeks. Even after the worst symptoms pass, your appetite and energy levels may take a few extra days to fully return.
Why the Diarrhea and Vomiting Happen
Your intestines normally absorb water and nutrients from everything you eat and drink. A stomach virus disrupts that process by interfering with how cells in your intestinal lining move water and salts. Instead of absorbing fluid, your gut starts secreting it, and in some cases the virus shuts down your intestine’s ability to absorb at all. The result is the watery diarrhea that defines the illness. Vomiting happens because the infection triggers your body’s defense response, trying to expel the virus as quickly as possible.
Viral vs. Bacterial: How to Tell the Difference
Most stomach bugs are viral, but bacterial infections (sometimes called food poisoning) can look similar at first. A few differences help separate them.
Viral gastroenteritis almost always produces watery, non-bloody diarrhea. Bacterial infections caused by organisms like Salmonella or Shigella are more likely to cause high fever, bloody diarrhea, and more severe exhaustion. If your stools contain visible blood or mucus, that points more toward a bacterial cause.
Some bacterial infections, like those from Staph toxins, actually mimic the viral pattern closely: sudden onset of watery diarrhea and vomiting with little or no fever. The key difference is timing. Toxin-based food poisoning often hits within hours of eating contaminated food, while a viral stomach bug takes a day or two to develop symptoms.
Dehydration: The Biggest Risk
The virus itself is rarely dangerous. Dehydration is the real threat, especially for young children, older adults, and people with weakened immune systems. When you’re losing fluid from both ends, your body can fall behind quickly.
Signs of dehydration in adults include:
- Extreme thirst and a dry mouth
- Dark-colored urine or urinating much less than usual
- Feeling lightheaded or faint
- Fatigue that feels disproportionate to the illness
- Sunken eyes or cheeks
- Skin that stays “tented” when you pinch it instead of flattening back immediately
Sipping small amounts of fluid frequently works better than drinking large amounts at once, which can trigger more vomiting. Water, broth, and oral rehydration solutions all help replace lost fluids and electrolytes.
Symptoms in Babies and Young Children
Children get the same core symptoms as adults, but they dehydrate faster because of their smaller body size. A baby who normally produces 6 to 8 wet diapers in 24 hours is a concern if they go 8 to 12 hours without a wet diaper. Other warning signs include crying without tears, a dry mouth, unusual drowsiness or fussiness, and being difficult to wake up.
Young children may not be able to describe what they’re feeling, so watch their behavior. A child who is less active than normal, refuses fluids, or seems “strangely drowsy” needs closer attention. Norovirus alone causes roughly 465,000 emergency department visits per year in the U.S., mostly in young children.
When Symptoms Signal Something More Serious
Most stomach bugs resolve on their own. Certain symptoms, however, suggest a more serious infection or dangerous level of dehydration:
- Diarrhea lasting more than 2 days
- Six or more loose stools in a single day
- High fever
- Bloody or black, tarry stools
- Severe abdominal or rectal pain
- Frequent vomiting that prevents you from keeping any fluids down
- Confusion, irritability, or unusual lack of energy
Bloody diarrhea in particular is a red flag. It usually indicates a bacterial infection or, in rare cases, a complication that needs medical evaluation. Black or tarry stools can signal bleeding higher in the digestive tract.
Adults aged 65 and older face the highest risk of serious outcomes. Norovirus causes roughly 900 deaths per year in the U.S., the vast majority in this age group. For older adults, even moderate dehydration can cascade into more serious problems, so early and consistent fluid replacement matters.