Noroviruses are a group of related viruses. Infection with these viruses affects the stomach and intestines and causes an illness called gastroenteritis (inflammation of the stomach and intestines).
Anyone can be infected with noroviruses and get sick. Also, you can get norovirus illness more than once during your life. The illness often begins suddenly. You may feel very sick, with stomach cramping, throwing up, or diarrhea.
Noroviruses are the most common cause of gastroenteritis in the United States. It is estimated that more than 20 million cases of acute gastroenteritis are caused by noroviruses annually--or about 1 in every 15 Americans will get norovirus illness each year. Norovirus is also estimated to cause over 70,000 hospitalizations and 800 deaths each year in the United States.