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Paralytic polio can be introduced into a community with a high level of immunity either by an infected, previously unvaccinated individual or by spread of attenuated vaccine virus that has mutated and become neurovirulent. Thanks to effective vaccine campaigns, in most of the world wild poliovirus disease has disappeared. However, small pockets of poliovirus disease exist in high-risk locations where vaccination programs have been less successful. Of the three types of poliovirus (types, 1, 2, and 3), type 2 poliovirus apparently has disappeared: The last naturally occurring case occurred in India in 1999. Therefore, the World Health Organization recommended withdrawal of trivalent oral poliovirus vaccine and replacing it with bivalent (typ…