Polio is defined as an infection caused by the Poliomyelitis enterovirus. The majority of infections in Polio are characterised by a mild fever, usually with diarrhea or vomiting. In a minority of subjects, weakness or complete paralysis of any of the skeletal muscles appears , although after a few days or weeks, the weakness begins to improve and may continue to do so for a few years. In the most severely Polio affected patients, however, all the trunk and limb muscles, and the swallowing and breathing muscles, may be affected and to maintain life, the treatment with mechanical respiratory support is required. After World War 2, Polio occurred in widespread epidemics, but with the introduction in the late 1950's of effective immunisation these epidemics have largely disappeared.