Noroviruses in the gastrointestinal tract. Noroviruses are a group of related, single-stranded RNA, non-enveloped viruses that cause acute gastroenteritis in humans. Noroviruses were previously known as Norwalk-like viruses(NLV). Noroviruses spread from person to person through contaminated food or water, and by touching contaminated surfaces. It is recognized as the leading cause of nonbacterial foodborne-disease outbreaks in the United States. Also shown are cross-sections of villa and epithelial cells.