Poliovirus is a type of enterovirus, meaning it enters through the oropharynx (mouth and nasal cavities) and then replicates in the submucosal tissues of the pharynx and the gastrointestinal tract. It is a small virus (27-30 nm) that lacks a viral envelope but has a capsid that surround its single-stranded, positive-sense RNA genome, which is about 7,500 nucleotides long. viremia occurs in 4-8% of individulas, and causes minor illness, including headache, sore throat, and fever. In rare cases, less than 1%, the viremia is persistent enough that the virus can enter the central nervous system. The three serotypes are thought to differ only slightly in structure. Structurally, it was been found that there are slight differences in the capsid proteins of each serotype. It is unknown whether these structural differences contribute, but it has also been observed that there are differences in virulence among the three serotypes. PV1 (also known as Brunhilde) is the most common form encountered in nature, large epidemics of poliomyelitis are usually associated with serotype 1.