updated 8/14/2009 1:24:49 PM ET 2009-08-14T17:24:49

Polio is spreading in Nigeria and health officials say in some cases it's caused by the vaccine used to fight the paralyzing disease.

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In July, the World Health Organization issued a warning that this particular virus might extend beyond Africa. So far, 124 Nigerian children have been paralyzed this year — about twice those afflicted in 2008.

Nigeria and most other poor 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 small risk of causing polio. In even rarer instances, the virus in the vaccine can mutate into a deadlier version that ignites new outbreaks.

A virologist with the U.S. Centers for Disease Control and Prevention says genetic analysis proves such mutated viruses have caused at least seven separate outbreaks in Nigeria.

The vaccine used in the United States and other Western nations is given in shots, which use a killed virus that cannot cause polio.

Copyright 2009 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.

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