Classicly

It is a contagious disease in which the gray matter of the spinal cord is attacked, producing destruction of certain cells and causing paralysis. The disease is caused by a virus—tiny “bugs” smaller than bacteria. Apparently these “bugs” enter through the mouth on food or in water that has been contaminated in some fashion, or through the nose in breathing. It is not conclusively known how the disease is spread. Many people harbor the virus without any reaction whatsoever. Many others have mild symptoms which they disregard as a summer cold. Once symptoms have developed, special laboratory studies made by experts are usually carried out to establish the diagnosis accurately.