Table 1.

Clinical and Epidemiologic Features of Norovirus Infections.

Variable	Description
Age predisposition	Affects all ages.
Seasonality	Occurs year round; outbreaks tend to peak in cold weather.
Settings	Outbreaks often occur in semi-closed environments (e.g., nursing homes, hospitals, cruise ships), the military, and schools and at recreational activities (e.g., sports events, camping trips, travel) that favor person-to-person spread.
Incubation period	10–51 hr.
Symptoms	Sudden onset of vomiting (more common in children) and diarrhea (more common in adults). Diarrheal stools usually contain no blood, mucus, or leukocytes. Asymptomatic infections are estimated to occur in approximately one third of infected persons.
Severity of illness	Overall, less severe than many other diarrheal infections but can lead to dehydration and hospitalization, especially among children <5 yr of age and adults >65 yr of age.
Duration of illness	Typically 28–60 hr; longer than 3 days in 15% of cases; longer illness in immunocompromised persons and adults with underlying illnesses.
Viral shedding	Peaks 1–3 days after onset of illness. Recent data indicate that viral antigen may be shed for up to 56 days. Shedding can be prolonged in immuno-compromised persons. Shedding may precede illness.
Mode of transmission and vehicles	Fecal–oral; aerosol–vomitus; contact with fomites; food, water, or environmental contamination; foods can be contaminated at the source (e.g., oysters, raspberries) or during preparation by food handlers.
Immunity	Illness results in short-term homologous immunity; infections can occur with other strains or with the same strain later in life; repeated exposure may generate long-term immunity.
Treatment	Supportive therapy to prevent dehydration; no specific antiviral therapy available. Vaccine development is in early phase.
Reservoir	Humans; recent evidence raises possibility of animal reservoir.