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- Small, non-enveloped, single-stranded positive-sense RNA virus.
- Noroviruses are 1 of 5 genera in the Caliciviridae family.
- Sapoviruses are another genus in the family that also cause acute gastroenteritis.
- Noroviruses are grouped into genogroups (I, II, and IV cause human infections), further divided into genetic clusters.
- Genogroup II, genotype 4 (II.4) is most commonly associated with outbreaks and severe disease.
- Genogroup I generally causes less severe disease and a lower attack rate partly because of an association between ABO blood type and infection.
- Previously known as "Norwalk-like viruses."