Why do polio infections sometimes cause paralysis?

Most polio infections cause mild or no symptoms, but in a small fraction of cases the virus reaches the central nervous system. There it can damage anterior horn motor neurons, which control voluntary movement. If enough of these nerve cells are lost, the result is acute flaccid paralysis, sometimes affecting breathing muscles. Widespread vaccination led to a dramatic global decline in polio cases, and the disease is now endemic in only a few places. The inactivated polio vaccine and oral polio vaccine have both played major roles in eradication efforts.