Most viral gastrointestinal infections clear up on their own, but you should see a doctor for severe cases. For adults, a serious case is indicated by blood in the diarrhea or vomit; vomiting for more than 24 hours,; or by signs of dehydration such as dark colored urine, weakness, dryness in the mouth, and dizziness.
Children and infants, more susceptible to severe cases, should be monitored closely. Take them to see a doctor if there is blood in their diarrhea or vomit; if they have been vomiting for more than 12 hours; or if they show signs of dehydration. Infants may show additional signs of dehydration such as not being able to produce tears when they cry or having a sunken soft spot on top of their heads.