Growth defect in polio. Coloured X-ray of a section through the legs of a 35-year-old male patient, showing a significant difference in leg length due to a major tilt in the pelvis caused by polio infection as a child. Polio (poliomyelitis) is caused by infection with the poliovirus. The virus attacks the motor neurons in the brain and spinal cord, and causes severe disability and paralysis and sometimes death.