Multiple sclerosis (MS) is a chronic disease that affects the central nervous system. MS is thought to be an autoimmune disease. In MS, the bodyÃƒÂ¢Ã¢â€šÂ¬Ã¢â€žÂ¢s immune system produces cells and proteins (antibodies) that attack myelin, a fatty substance that protects nerve fibers. The cause of MS is unknown. It is not an inherited disease, but it appears that genetic factors play a role in making some people more susceptible to developing it.