Poliovirus: A member of the enterovirus subgroup, family Picornaviridae. Enteroviruses are transient inhabitants of the gastrointestinal tract. Picornaviruses are small viruses with an RNA genome. There are three strains of poliovirus. They are called P1 or the Brunhilde virus, P2 or the Lansing virus, and P3 or the Leon virus). Immunity to one strain of poliovirus does not produce significant immunity to the other two strains. Poliovirus is rapidly inactivated by heat, formaldehyde, chlorine, or ultraviolet (UV) light.