According to the Centers for Disease Control and Prevention, a norovirus is a very contagious virus that causes gastroenteritis (inflammation of the stomach and/or intestinal lining), as a result of food poisoning or infected food, and can develop into acute non-bacterial gastroenteritis. It spreads rapidly from an infected person, contaminated water or food, or by touching contaminated surfaces. It causes your stomach or intestines (or both) to become inflamed, causing stomach pain, nausea, and diarrhea, and often vomiting. According to research, it appears that norovirus is responsible for about 50 percent of all gastroenteritis outbreaks in the United States and about 90 percent of all epidemic nonbacterial gastroenteritis outbreaks worldwide.