A stomach bug typically shows up as sudden, watery diarrhea paired with nausea or vomiting, abdominal cramps, and sometimes a low-grade fever. Most people feel the worst during the first 24 hours, and the whole illness runs its course in one to three days. Here’s what to expect at each stage and how to tell if something more serious is going on.
The Core Symptoms
Viral gastroenteritis, commonly called a stomach bug or stomach flu, hits with a recognizable pattern. The hallmark symptoms are watery diarrhea (not bloody), nausea or vomiting (often both), and cramping pain in your abdomen. You may also notice muscle aches, a headache, and a mild fever. The diarrhea is loose and frequent, and it’s almost always non-bloody. If you see blood in your stool, that points to a different and more serious type of infection, not a typical stomach virus.
Vomiting often comes first and can be intense for the first several hours. Some people vomit repeatedly before the diarrhea starts, while others experience both at the same time. The abdominal pain tends to come in waves, cramping before a bout of diarrhea and easing temporarily afterward.
How It Looks in Your Body Over Time
After you’re exposed to a stomach virus like norovirus, symptoms appear within 12 to 48 hours. The onset is sudden. You might feel perfectly fine in the morning and be unable to leave the bathroom by evening. That rapid shift from “normal” to “miserable” is one of the most distinctive features of a stomach bug.
The first 12 to 24 hours are usually the worst. Vomiting is most frequent during this window, and diarrhea can send you to the bathroom six or more times a day. By day two, the vomiting typically slows or stops, though diarrhea may linger. Most people feel significantly better within one to three days. In some cases, particularly in young children, older adults, or people with weakened immune systems, symptoms can stretch out to 14 days.
One important detail: even after you feel better, you can still spread the virus for two weeks or more. That’s why hand-washing matters well beyond the point where your symptoms disappear.
Physical Signs of Dehydration
The biggest visible concern with a stomach bug isn’t the vomiting or diarrhea itself. It’s the water and electrolytes your body loses in the process. Dehydration is the main complication, and it’s something you can spot by looking for a few telltale signs.
Dark yellow or amber-colored urine is one of the earliest indicators. If you’re barely urinating at all, dehydration has progressed further. Dry mouth, dry lips, and feeling lightheaded when you stand up are other clear signals. In babies and young children, watch for fewer wet diapers than usual, crying without tears, and unusual sleepiness or fussiness. A sunken soft spot on an infant’s head is a serious sign that they need fluids right away.
Stomach Bug vs. Food Poisoning
These two conditions look almost identical on the surface, but their timing is the giveaway. A stomach bug takes 24 to 48 hours after exposure before symptoms appear. Food poisoning hits much faster, typically within two to six hours of eating contaminated food. Your body is essentially trying to expel whatever it took in.
Food poisoning also tends to resolve more quickly. It’s often over within a day, whereas a stomach bug lingers for two to three days. If several people who ate the same meal all get sick within hours, that’s likely food poisoning. If family members get sick one after another over several days, a virus is the more probable cause.
What It Looks Like in Kids
Children, especially toddlers and infants, often get hit harder by stomach bugs. Vomiting may be more frequent and dramatic, and young kids have a harder time keeping any fluids down. They can become dehydrated faster than adults because of their smaller body size. Frequent vomiting in an infant or young child is a reason to contact a pediatrician right away, even if the child doesn’t seem severely ill otherwise.
In older kids, the symptoms look similar to adults: cramping, watery diarrhea, and vomiting that peaks in the first day. Children may also spike a higher fever than adults and seem unusually lethargic or clingy. Most kids bounce back within a couple of days, but keeping them hydrated with small, frequent sips is critical during the worst of it.
Signs That Need Medical Attention
Most stomach bugs are unpleasant but harmless. A few specific signs, however, mean the illness has moved beyond what your body can handle on its own. Blood in your stool or vomit is never normal with a standard stomach virus and warrants a call to your doctor. The same goes for a fever above 101.5°F, or if you can’t keep any liquids down for 24 hours straight.
Head to an emergency room if your temperature climbs above 102°F for more than two days and doesn’t respond to fever-reducing medication, if your urine turns very dark or stops entirely, or if you feel dizzy and lightheaded every time you stand. These are signs of severe dehydration that may need intravenous fluids. For adults, vomiting or diarrhea lasting more than two to three days also crosses the line into “get checked out” territory.