At one time, polio was one of the most dreaded childhood illnesses of the 20th century. The disease caused severe illness in thousands of people each year before the vaccine was introduced in 1955. Thanks to the introduction and widespread use of the vaccine, the number of cases has decreased significantly.
Polio is caused by a virus and is transmitted easily from person to person. Up to 95 percent of people infected with poliovirus have no symptoms. When symptoms do occur, they can range in severity from minor (such as nausea or flu-like symptoms) to major problems (such as paralysis).
(To learn more about this disease and its effects within the body, click Polio.)