Norovirus virion, artwork. This virus was previously known as the Norwalk virus. A single virus particle (virion) is shown here, with a sphere of proteins (pentons, red; hexons, black) forming an external shell known as the capsid. Inside the capsid is the genetic material, ribonucleic acid (RNA, green). One of the capsid proteins has been pulled outwards (left), showing how the capsid proteins encapsulate the RNA. Noroviruses are highly contagious and are transmitted via faecally contaminated food and water, and by person-to-person contact. They cause vomiting and diarrhoea, and are a major cause of acute gastroenteritis worldwide.