 Poliovirus , the causative agent of poliomyelitis ( commonly known as polio ) , 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 and has a positive sense 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