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Norovirus is a genus in caliciviridae and includes a single species called norwalk virus. Human noroviruses are major pathogens of acute gastroenteritis, especially in winter, and are transmitted via a fecal-oral route, either by ingestion of food or water contaminated with feces or by direct or indirect contact with the vomit or feces of an infected person. The number of patients infected with noroviruses has accounted for half of all of the patients with food poisoning in recent years in Japan. Epidemiological features of norovirus outbreaks have changed in recent years:for instance, foodborne outbreaks associated with foods contaminated from an infected food handler other than bivalve mollusks such as oysters have increased, and outbreaks in elderly facilities have increased. New variant strains of GII.4 have emerged every year or every few years because the genome of GII.4 changes more frequently than the genomes of other genotypes. The emergence of GII.4 2006b and GII.4 Sydney 2012 variants led to many outbreaks in the 2006/07 and 2012/13 seasons, respectively. Point mutations and recombinations in the genome may be responsible for the evolution of noroviruses. Noroviruses have been detected not only in humans but also in animals such as cattle, pigs and mice. There is no direct evidence that noroviruses cause zoonotic transmission, but some reports have indicated that possibility. There is a need for an approach from the viewpoint of zoonosis in the future studies.
