Norwalk virus particle. Computer artwork of the capsid of the Norwalk virus with proteins represented by coloured blobs. The capsid is a protein coat that encloses the virus's RNA (ribonucleic acid) genome. This capsid is icosahedral in shape, with 20 triangular faces. The Norwalk virus belongs to the family Calciviridae, which are small viruses, lacking an outer glycoprotein coat. The virus is transmitted via contaminated water and food, and causes vomiting and diarrhoea.