Norwalk virus infection. Computer artwork of a norwalk virus and a human gut. The stomach is at top, the large intestine goes around the outside of the virus, which is covering most of the small intestine. The norwalk virus is a small virus consisting of an RNA (ribonucleic acid) core surrounded by a protein capsid (multicoloured). Infection with the virus is very common and causes vomiting, diarrhoea and stomach pains. It is transmitted via contaminated food and water.