Human caliciviruses are often observed in patients with diarrheal illness. The best known of the human agents is Norwalk virus, which is a major cause of epidemics of self-limited diarrhea and vomiting in school children and adults. Resistance to the infection is unrelated to serum or local gut antibody. This common illness has an abrupt onset and is accompanied by a varying combination of signs and symptoms, including abdominal cramps, myalgias, malaise, headache, nausea, and low-grade fever. The disease usually resolves spontaneously within 24 to 48 hours.
Fatalities are very rare and are confined to elderly or debilitated individuals.
Some other human caliciviruses produce a clinical syndrome similar to that of Norwalk virus, but, similar to rotavirus gastroenteritis, some virus strains also cause diarrhea and vomiting in infants and young children. The virion contains single-stranded, positive sense RNA of the size of about 7.5 kb. Norwalk virus is recognized in human stool specimens by immune electron microscopic or immunoassay techniques. On the basis of their morphology, size, protein profile, and nucleic acid, Norwalk virus and other human caliciviruses are classified in the family Caliciviridae. Several viruses are described that share virologic characteristics with Norwalk virus as well as association with epidemics or family outbreaks of acute gastroenteritis.