Most stomach bugs cause a day or two of vomiting and diarrhea that settle with rest and small frequent sips of fluid. The main danger is dehydration, especially in young children, older adults, and people with other health conditions.
Seek urgent medical care if you cannot keep any fluids down for more than a day, see blood or black material in vomit or stool, have severe or constant abdominal pain, a high fever, or signs of dehydration such as very dark or little urine, extreme thirst, dizziness, a dry mouth, sunken eyes, or confusion. Green or bloody vomiting, a swollen tender belly, or symptoms following travel to high-risk regions also need prompt assessment.
Oral rehydration solutions help replace lost fluids and salts and are preferable to water alone in significant illness. If symptoms are severe, persistent beyond a few days, or accompanied by any red flag, contact a doctor or seek in-person care rather than waiting it out.