Polio (or poliovirus) is an enterovirus that causes poliomyelitis, commonly known as polio. The virus enters the body through the mouth (it can be carried in fecal matter) and multiplies in the throat and intestines. From there it invades the nervous system. Its usual symptoms include paralysis, and it can be very fast-acting. However, it can also have no symptoms at all and thus be transmitted by people who don't know they have it.
Now very rare in the Western hemisphere, polio is still a major problem in developing countries of the Eastern hemisphere. Since children under the age of five are most susceptible to the disease caused by poliovirus, universal immunization by vaccine is the best way to stop the disease. There is no cure for the disease once contracted.