Polio is a disease caused by a virus. It enters a child’s (or adult’s) body through the mouth. Sometimes it does not cause serious illness. But sometimes it causes paralysis (can’t move arm or leg). It can kill people who get it, usually by paralyzing the muscles that help them
breathe.

Polio used to be very common in the United States. It paralyzed and killed thousands of people a year before we had a vaccine for it.