The Polioviruses
The polioviruses are three related enteroviruses: Types 1, 2 and 3. All three types cause paralysis. Type 1 causes paralysis most frequently. The type 2 virus has not been detected worldwide since October 1999. Type 3 virus is now isolated very infrequently in India.

Immunity
Protective immunity against poliovirus infection develops by immunization or natural infection. Immunity to one poliovirus type does not protect against other poliovirus types. Immunity following natural infection or by live oral polio vaccine (OPV) is believed to be lifelong. Infants born to mothers with high antibody levels against poliovirus are protected for the first weeks of life.

Occurrence
Poliomyelitis occurred worldwide, but is not limited to Asia and African continents. During 2003 the disease was endemic in 6 countries India, Nigeria, Niger, Pakistan, Afghanistan and Egypt. However importations from these endemic countries have been reported recently in other polio free countries and states. The disease is seasonal, with cases starting to increase sharply in June with peaks during July through September. Cases continue to be high in areas with low immunization coverage and poor sanitation.

Reservoir
Poliovirus infects only human beings and there is no animal reservoir.

Clinical spectrum
Inapparent (sub clinical) infection. This occurs approximately in 95 per cent of poliovirus infection.

Abortive polio or minor illness: Occurs in approximately 4 to 8 per cent of the infections.
Non paralytic polio: Occurs in approximately 1 per cent of all infections.
Paralytic polio: Occurs in less than one per cent of infections.