Norovirus is a highly contagious virus that causes acute gastroenteritis (inflammation of the stomach and intestines). Norovirus infection can spread quickly in closed and crowded environments such as hospitals, nursing homes, daycare centers, 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. Every year, it causes an estimated 19–21 million cases of acute gastroenteritis. Norovirus is not a reportable disease in California, therefore data are not available from the California Department of Public Health.
While it is possible to become infected with norovirus year round, norovirus infection happens most often from November to April. You can get it more than once because there are many different strains. There can be a higher incidence of norovirus illnesses in years when there is a new strain of the virus going around.
Norovirus is found in the feces and vomit of infected people and can spread easily from person to person. People can become infected in several ways, including:
having direct contact with another person who is infected (for example, caring for or sharing foods or eating utensils with someone who is ill)
eating food or drinking liquids contaminated with norovirus, such as food touched by an ill food handler or undercooked seafood that has been harvested from contaminated waters
touching contaminated surfaces or objects and then touching their mouth before hand washing