Poliomyelitis, commonly known as polio, is a highly contagious viral infection that can cause paralysis and even death. It is caused by the poliovirus, which is spread through contact with an infected person's saliva or feces. The virus attacks the nervous system, leading to muscle weakness and paralysis. Polio was once a major public health concern in the United States, but thanks to the development of a vaccine in 1955, the disease has been virtually eliminated in the U.S. However, it still affects many countries around the world.