Polio is caused by a virus that enters the body through the intestinal tract and can extend into the blood stream. From the blood, it enters, infects, and destroys the nerve cells of the spinal cord and brain stem (lower part of the brain). Paralysis and even death can result. Polio was a common, devastating disease causing lifelong suffering until an effective vaccine was made widely available in the 1950s. Since then, the number of polio cases has plummeted. The only polio seen now in the United States is associated with the oral, live virus vaccine.