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Gastroenteritis is one of the most common causes of morbidity and mortality in children worldwide and accounts for 2 to 3 million deaths per year with most occurring in young children in developing countries.1The causes of gastroenteritis in children vary with the location, time of year, and population studied. Bacterial and parasitic gastrointestinal infections have decreased in frequency as a result of the provision of safe drinking water and disposal of sewage. However, viral gastroenteritis has not declined in a comparable fashion.Viruses are the most common pathogens causing gastroenteritis in the United States.2 Before universal rotavirus immunization was adopted in the United States, approximately one half of all hospitalizations for nonbacterial gastroenteritis in children were caused by rotaviruses. Currently, noroviruses are the most important cause of nonbacterial acute gastroenteritis. In children, other viral pathogens that have been proven to cause gastroenteritis are sapoviruses, astroviruses, and enteric adenoviruses. The viral gastroenteritis pathogens all infect the intestine and cause gastrointestinal symptoms. In contrast, enteroviruses may infect the intestine but predominantly produce manifestations at extraintestinal sites.Caliciviruses are a family of single-stranded, nonenveloped RNA viruses. The 2 recognized genera that cause disease are noroviruses (Norwalk-like viruses) and sapoviruses (Sapporo-like viruses).3 Caliciviruses have a worldwide distribution, with multiple antigenic types circulating simultaneously in the same region.Noroviruses are the leading cause of outbreaks of gastroenteritis and an important cause of sporadic gastroenteritis.4 Most sporadic infections have been detected in children younger than 4 years of age.4In the United States, more than 90% of the outbreaks for which the cause would previously have been unknown can now be attributed to norovirus.5 These outbreaks involve people of all ages, occur in a wide variety of settings, such as nursing homes, hospital wards, day-care centers, cruise ships, restaurants, and catered events, and target a number of high-risk groups, particularly young children and the elderly, travelers, soldiers, and immunocompromised patients.Noroviruses are transmitted by the fecal-oral route, either by consumption of contaminated food or water or by direct person-to-person spread. Environmental contamination may also act as a source of infection. Noroviruses are highly contagious.6 An inoculum of as few as 10 viral particles may be sufficient to infect an individual. The incubation period usually is 24 to 48 hours.Norovirus infection presents with acute onset of vomiting, watery nonbloody diarrhea with abdominal cramps, and nausea. Low-grade fever also occasionally occurs. Vomiting is more common in children. Dehydration is the most common complication, especially among the young. Symptoms usually last for 24 to 60 hours.In the last 10 years, the diagnosis of outbreaks has improved with the increasing use of a norovirus RT-PCR which can be used to test stool and emesis samples, as well as to detect the presence of noroviruses on environmental swabs. This test is available through the state public health laboratories in all 50 states.No specific therapy exists for norovirus gastroenteritis. Symptomatic therapy consists of replacing fluid losses and correcting electrolyte disturbances.Sapoviruses mainly infect young children. The illness is milder than that caused by noroviruses.3 Antibody prevalence studies show that virtually all children are infected with sapoviruses by the time they are 5 years of age, indicating that sapovirus infection is widespread, although the illness most likely is sporadic with a high rate of asymptomatic infection. The transmission of sapoviruses is thought to occur by the fecal-oral route with an incubation period of 24 to 48 hours. Sapovirus infections occur year-round, although a winter seasonal peak has been suggested.