Most of the time, polio has no symptoms unless the polio virus gets into the blood. It is uncommon for the virus to enter the brain or spinal cord. If this does happen, it can cause muscles to become paralyzed. Some people get better from the paralysis. Others will be disabled. Depending on which muscles have been affected, these people may need a mobility aid or a wheelchair; they may have difficulty using their hands; or they may even have trouble breathing. 