Multiple sclerosis (MS) is an unpredictable disease of the nervous system that disrupts communication between the brain and other parts of the body. Its effects can range from relatively mild in most cases to somewhat disabling to devastating. The symptoms may mysteriously occur and then disappear. In the worst cases, a person with MS may be unable to write, speak, or walk. About 350,000 Americans have MS, with most cases occurring between 18 - 45 years of age.