The picornaviridae comprise four genera — enteroviruses, cardioviruses, rhinoviruses, and aphthoviruses — which have many similarities in structure and genome organization. 1 While hepatitis A has proved difficult to characterize (because of its relatively recent identification, restricted host range, and limited growth in cell culture), it is clearly a picornavirus and has been finally classified as an enterovirus. 2 although it has some distinguishing characteristics from other members of this genus (see Chapter 3).