Polio (poliomyelitis) is a disease caused by poliovirus. It causes mild or no symptoms in most people, but in some people it can cause paralysis or death. There are three variations of poliovirus, called wild poliovirus type 1, 2 and 3 (WPV1, WPV2 and WPV3). Wild polio types 2 and 3 have been eradicated (no longer exist), and wild polio type 1 only exists in a few parts of the world. Polio type 1 is most likely to cause paralysis.