All were eventually eliminated by immunizing more children. Experts said that the most recent outbreak was able to spread because, until recently, only 30 percent to 40 percent of the children in northern Nigeria were vaccinated. About 70 percent are vaccinated there now, Aylward said.

In 2000, the United States switched to injected vaccine made from killed virus, which cannot mutate. But oral drops with the live, weakened version of the virus are still used in most poor countries, including those where the disease has never been eliminated: Nigeria, India, Pakistan and Afghanistan.

This vaccine, invented by Albert Sabin, is easier to give, offers much stronger protection and can be...

All were eventually eliminated by immunizing more children. Experts said that the most recent outbreak was able to spread because, until recently, only 30 percent to 40 percent of the children in northern Nigeria were vaccinated. About 70 percent are vaccinated there now, Aylward said.

In 2000, the United States switched to injected vaccine made from killed virus, which cannot mutate. But oral drops with the live, weakened version of the virus are still used in most poor countries, including those where the disease has never been eliminated: Nigeria, India, Pakistan and Afghanistan.

This vaccine, invented by Albert Sabin, is easier to give, offers much stronger protection and can beneficially "infect" other family members or neighbors, protecting them too.

But in rare cases, it can mutate into something resembling wild polio virus, which can paralyze or kill. Aylward pointed out that 10 billion doses of oral vaccine had been given in the past 10 years, so such mutations are presumably extremely unusual.

Polio often circulates undetected. In only one of 200 infections will it cause paralysis, which signals health officials to look for the virus in the area.