Advances in the 1950s and 1960s, including unprecedented cooperation between Soviet and U.S. scientists, allowed polio to be eradicated throughout the Americas by 1994 and all of Europe in 1998. Eliminating the crippling scourge has been more difficult, however, in some parts of Africa and Asia.

Sources:
http://www.nytimes.com/2011/02/01/health/01polio.html
http://americanhistory.si.edu/polio/timeline/index.htm
http://www.polioeradication.org/Aboutus/History.aspx
www.fda.gov/AboutFDA/WhatWeDo/History/FOrgsHistory/CBER/ucm135758.htm
http://www.marchofdimes.com/mission/history_indepth.html
https://www.npr.org/templates/story/story.php?storyId=4585992