Poliovirus, the causative agent of poliomyelitis, is a human enterovirus and member of the family of Picornaviridae. Poliovirus is composed of an RNA genome and a protein capsid. The genome is a single-stranded positive-sense RNA genome that is about 7500 nucleotides long.The viral particle is about 30 nanometres in diameter with icosahedral symmetry. Because of its short genome and its simple composition-only RNA and a nonenveloped icosahedral protein coat that encapsulates it-poliovirus is widely regarded as the simplest significant virus.