Norwalk virus and related "Norwalk-like viruses" are the most common cause of outbreaks of stomach sickness (nausea, vomiting, diarrhea) in older children and adults in the United States. These viruses are sometimes found in drinking water, ice, shellfish and in other foods. They can be spread easily from contact with water, food, objects or hands that have even small amounts of feces from someone who was sick.