When individuals refer to having “the stomach flu,” they are likely describing acute-onset gastroenteritis caused by noroviruses, which belong to the Caliciviridae family. Importantly, noroviruses are entirely distinct from influenza viruses.
According to the Centers for Disease Control and Prevention (CDC), noroviruses are responsible for nearly 21 million cases of acute gastroenteritis each year, making them the leading cause of gastroenteritis in adults in the United States. Norovirus is highly contagious, spreading rapidly from infected individuals. Its genetic simplicity allows for rapid evolution, and exposure does not confer lasting immunity. On average, individuals are estimated to experience norovirus infections approximately five times throughout their lives.
Norovirus exhibits a unique structure, resembling a round blue ball composed of protein encasing its genetic material. The virus attaches to the intestinal lining’s surface cells, transferring its genetic material into those cells. Once inside, norovirus replicates, ultimately killing the host cells and releasing new virus particles that can infect additional intestinal cells.
Due to its stable environmental structure, norovirus is resistant to many sanitizers and disinfectants, posing significant challenges for sectors such as healthcare, education, tourism, and food safety, particularly regarding shellfish and produce. Most foodborne norovirus outbreaks are linked to restaurants or institutional food service settings.
Norovirus is currently the most prevalent cause of acute gastroenteritis in the United States. While bacterial sources of foodborne illness, like pathogenic Escherichia coli and Salmonella, receive notable attention, noroviruses account for approximately 58% of foodborne illnesses in the U.S., making them the leading cause of outbreaks from contaminated foods. CDC estimates suggest there are around 2,500 reported norovirus outbreaks each year in the United States. Although norovirus outbreaks can occur year-round, they are most frequent from November to April. Approximately 1 in 15 individuals in the U.S. contracts norovirus illness annually. By the age of five, 1 in 14 children will visit an emergency room, and 1 in 278 will be hospitalized due to norovirus.
Outbreaks have been reported in various settings, including healthcare facilities, restaurants, catered events, schools, and childcare centers. Notably, cruise ships account for only a small percentage (1%) of reported norovirus outbreaks overall. In addition to humans, norovirus can infect a wide range of hosts, including livestock, pets, and various wild animals.