Rotavirus is a genus of double-stranded RNA viruses in the family Reoviridae. Rotaviruses are the most common cause of diarrhoeal disease among infants and young children. Nearly every child in the world is infected with a 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 nine species of the genus, referred to as A, B, C, D, E, F, G, H and I. Rotavirus A, the most common species, causes more than 90% of rotavirus infections in humans.