Question: when do you know if you have a stomach virus?
Answer: Symptoms. Gastroenteritis causes diarrhea, nausea, and possibly vomiting. You also may have a headache, fever, chills, and pain in your belly. The most common kind -- the one caused by a virus -- makes you feel sick 12 to 48 hours after you're exposed to it, and it can last up to 3 days.