Norovirus is a very contagious virus that causes acute vomiting and diarrhea. Norovirus can survive for weeks on surfaces and objects, such as countertops, doorknobs, phones, and furniture. Norovirus can spread quickly in closed and crowded places such as hospitals, nursing homes, schools, and cruise ships. It is sometimes called "stomach flu," but is not related to influenza (flu) viruses. Norovirus is the most common cause of acute gastroenteritis in the United States, causing an estimated 19 to 21 million cases every year. Norovirus infection can be prevented by washing hands with soap and water, cleaning contaminated surfaces and objects, and staying home when ill with vomiting and diarrhea.