Polio is spreading in Nigeria -- and health officials say in some cases it's caused by the very vaccine used to fight the paralyzing disease.
The World Health Organization has issued a warning that this particular virus might extend beyond Africa. More than 120 Nigerian children have been paralyzed this year -- twice the number afflicted in 2008.
Nigeria and many other nations use an oral polio vaccine because it's cheaper, easier, and protects entire communities. But it's made from a live polio virus which carries a risk of causing polio. In even rarer instances, the virus in the vaccine can mutate into a deadlier version that ignites new outbreaks. Genetic analysis has proven that such mutated viruses have caused at least seven separate outbreaks in Nigeria.