These are small round RNA viruses. There are some similarities to calciviruses. Infection predominately involves adults, adolescents, and older children. About 55%-75% of adults have antibodies against this virus. Third-world countries have a higher incidence of antibody. Epidemiologically, disease in the United States usually occurs in clusters (outbreaks); reviews found 34%-47% of such gastroenteritis outbreaks (cruise ships, schools, camps) were due to Norwalk viruses. Clinical disease is usually relatively mild and self-limited. Incubation appears to be about 24 hours (range, 10-50 hours). Nausea and vomiting are usually more prominent than diarrhea. The acute phase usually ends in 24-48 hours and most patients do not require hospitalization. The virus appears to affect the jejunum. Diagnosis has been difficult; until recently, stool EM was required. However, EM apparently has sensitivity of only 34%-48%. Even nucleic acid probes with PCR amplification appear to detect less than 85% of cases (using homemade reagents). Enzyme immunoassay methods using monoclonal antibody against Norwalk antigen in stool have also been reported using homemade reagents; these have generally detected less than 50% of cases. The organism has not been cultured to date.