Multiple sclerosis (MS) is a disease of the nervous system. The exact cause of MS is unknown, but it is believed to be an autoimmune condition. Autoimmune conditions are diseases that cause the body's immune system and natural defenses to attack healthy cells. In the case of MS, the immune system begins attacking myelin, the cells that make up the sheath covering nerves. Without myelin, nerves are unable to transmit signals effectively and symptoms occur.