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The eradication of smallpox is one of the greatest medical achievements. Polio is on the short list of infectious diseases that may be eradicated in the near future. In fact, transmission of poliovirus serotype 2 appears to have been interrupted globally already. However, transmission of serotypes 1 and 3 persists. In 2004, transmission of wild-type poliovirus continued in six countries. Transmission continues partly because of under-immunization, specifically in Nigeria, Niger, Pakistan, and Afghanistan, and failure of the trivalent oral polio vaccine (tOPV) in Egypt and India. Vaccine failure may stem partly from competing enteric infections, including infection with other vaccine serotypes. Because serotype 1 was the primary circulating …