Poliomyelitis is caused by poliovirus serotypes 1, 2 and 3. Polioviruses are neuroinvasive enteroviruses that are spread by the faecal-oral and respiratory routes. Most infections are sub-clinical, but a minority give rise to neurological manifestations including aseptic meningitis, encephalomyelitis, and the poliomyelitis syndrome, characterized by the acute onset of flaccid paralysis. The live attenuated oral poliovirus vaccine (OPV) is shed asymptomatically in the stool of vaccine recipients for several days; shedding may be prolonged in immunocompromised persons, which may allow reversion to virulence and give rise to vaccine-associated paralytic polio (VAPP) in vaccine recipients or their contacts.