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Girl affected by polio. Polio, or poliomyelitis, is an infection of the grey matter in the spinal cord by poliovirus. Although most cases show no symptoms, polio can cause atrophy (wasting away) and paralysis of the limbs. This photograph was part of a 1950s campaign to raise US public awareness of the need to vaccinate against polio. (KEYSTONE/SCIENCE PHOTO LIBRARY/CDC/SCIENCE PHOTO LIBRARY)