Rotavirus is the most common cause of severe vomiting and diarrhoea among infants and young children. It is a genus of double-stranded RNA virus in the family Reoviridae. Nearly every child in the world has been infected with rotavirus at least once by the age of five. Immunity develops with each infection, so subsequent infections are less severe; adults are rarely affected. There are eight species of this virus, referred to as A, B, C, D, E, F, G and H. Rotavirus A, the most common species, causes more than 90% of rotavirus infections in humans.