Multiple sclerosis is a chronic autoimmune disease in which the immune system attacks myelin, the insulating sheath around nerve fibers in the brain and spinal cord. Damage to myelin slows or blocks electrical signals between the brain and the body. MS affects approximately 1 million people in the United States and is one of the most common causes of non-traumatic neurological disability in adults aged 20 to 50. Women are about three times more likely than men to develop relapsing forms of the disease.