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The virus

Polio is caused by a human enterovirus called the poliovirus. Wild polioviruses are those that occur naturally.

The poliovirus consists of an RNA genome enclosed in a protein shell called a capsid. There are three serotypes of wild poliovirus – type 1, type 2, and type 3 – each with a slightly different capsid protein.

Scanning electron micrograph of the wild poliovirus
The wild poliovirus as seen through a microscope. The virus invades the nervous system, causing paralysis in one out of every 200 children.
From GPEI site photo gallery

Type 2 poliovirus has been eliminated in the wild – the last wild type 2 poliovirus was detected in India in 1999.

In this final stage of polio eradication, only type 1 and type 3 wild poliovirus continue to circulate in endemic areas. Both are highly infectious and both cause paralytic polio. Type 1 is the most pervasive strain of poliovirus and type 3 is at very low levels.