Poliomyelitis, often called polio or infantile paralysis, is an acute viral infectious disease spread from person to person, primarily via the fecal-oral route. Although around 90% of polio infections cause no symptoms at all, affected individuals can exhibit a range of symptoms if the virus enters the blood stream. In fewer than 1% of cases the virus enters the central nervous system, leading to muscle weakness and acute flaccid paralysis. Different types of paralysis may occur, depending on the nerves involved. Spinal polio is the most common form, characterized by asymmetric paralysis that most often involves the legs. Bulbar polio leads to weakness of muscles innervated by cranial nerves, difficulty in chewing, abnormal respiratory rate, depth, and rhythm, which may lead to respiratory arrest, shock, and may be fatal. There is no cure for polio. Two vaccines are used throughout the world to combat polio. Both vaccines induce immunity to polio. The Salk vaccine, or inactivated poliovirus vaccine (IPV). Oral polio vaccine (OPV) using live but weakened virus. Because OPV is inexpensive, easy to administer, and produces excellent immunity, it has been the vaccine of choice for controlling poliomyelitis in many countries. On very rare occasions (about 1 case per 750,000 vaccine recipients) the attenuated virus in OPV reverts into a form that can paralyze. Most industrial countries have switched to IPV. IPV vaccine is available as IMOVAX Polio, or as part of compound vaccine. A total of four doses are given before school entry. The fifth vaccination is given during adolescence.