A chronic, potentially debilitating disease that affects your brain and spinal cord (central nervous system). The illness is probably an autoimmune disease, which means your immune system responds as if part of your body is a foreign substance. In MS, your body directs antibodies and white blood cells against proteins in the myelin sheath surrounding nerves in your brain and spinal cord. This causes inflammation and injury to the sheath and ultimately to your nerves. The result may be multiple areas of scarring (sclerosis). The damage slows or blocks muscle coordination, visual sensation, and other nerve signals. The disease varies in severity, ranging from a mild illness to one that results in permanent disability. Treatments can modify the course of the disease and relieve symptoms. An estimated 400,000 Americans have MS. It generally first occurs in people between the ages of 20 and 50. The disease is twice as common in women as in men.