Structural model of a picornavirus particle, a member of one of a group of small RNA-containing viruses which includes Coxsackie viruses, echoviruses, polioviruses and rhinoviruses. Picornaviruses are small (20-30 nanometres diameter), possess icosahedral symmetry and have no envelope. The capsid houses 60 capsomere units, each composed of 4 polypeptides, and surrounds a single strand of RNA.