Norwalk virus capsid protein. Molecular model showing the secondary structure of the capsid protein from a norwalk virus particle. The protein is made up of three chains (centre left, top, and centre right). The capsid encloses the virus's genetic material RNA (ribonucleic acid). Infection with norwalk virus is very common and causes vomiting, diarrhoea and stomach pains. It is transmitted via contaminated food and water.