This transmission electron micrograph (TEM) revealed some of the ultrastructural morphology displayed by norovirus virions, or virus particles. Noroviruses belong to the genus Norovirus, and the family Caliciviridae. They are a group of related, single-stranded RNA, nonenveloped viruses that cause acute gastroenteritis in humans. Norovirus was recently approved as the official genus name for the group of viruses provisionally described as 揘orwalk-like viruses? (NLV).