does not involve the central nervous system (CNS), sometimes called abortive poliomyelitis, and a major illness involving the CNS, which may be paralytic or nonparalytic. Adults are more likely to develop symptoms, including severe symptoms, than children. In most people with a normal immune system, a poliovirus infection is asymptomatic. In about 25% of cases, the infection produces minor symptoms which may include sore throat and low fever. These symptoms are temporary and full recovery occurs within one or two weeks. In about 1 percent of infections the virus can migrate from the gastrointestinal tract into the central nervous system (CNS). Most patients with CNS involvement develop nonparalytic aseptic meningitis, with symptoms of headache, neck, back, abdominal and extremity pain, fever, vomiting, stomach pain, lethargy, and irritability. About one to five in 1000 cases progress to paralytic disease, in which the muscles become weak, floppy and poorly controlled, and, finally, completely paralyzed; this condition is known as acute flaccid paralysis. The weakness most often involves the legs, but may less commonly involve the muscles of the head, neck, and diaphragm. Depending on the site of paralysis, paralytic poliomyelitis is classified as spinal, bulbar, or bulbospinal. In those who develop paralysis, between 2 and 10 percent die as the paralysis affects the breathing muscles. Encephalitis, an infection of the brain tissue itself, can occur in rare cases, and is usually restricted to infants. It is characterized by confusion, changes in mental status, headaches, fever, and, less commonly, seizures and spastic paralysis. Cause Poliomyelitis is caused by infection with a member of the genus Enterovirus known as poliovirus (PV). This group of RNA viruses colonize the gastrointestinal tract – specifically the oropharynx and the intestine. The incubation time (form the first signs and symptoms) ranges from three to 35 days, with a more common span of six to 20 days. PV does not affect any species other than humans. Its structure is quite simple, composed of a single (+) sense RNA genome enclosed in a protein shell called a capsid. In addition to protecting the virus' genetic material, the capsid proteins enable poliovirus to infect certain types of cells. Three serotypes of poliovirus have been identified – wild poliovirus type 1 (WPV1), type 2 (WPV2), and type 3 (WPV3) – each with a slightly different capsid protein. All three are extremely virulent and produce the same disease symptoms. PV1 is the most