MS and MG (myasthenia gravis) are both in the autoimmune disease category. It is not uncommon for a person diagnosed with one autoimmune disease to be diagnosed with a second, or even a third.
In fact, the December 2010 issue of InFocus, the quarterly publication of the American Autoimmune Diseases Association (www.aaarda.org
) has a front page item from Carol Berger, of Horsham, Pennsylvania, who says, "… I myself have been diagnosed with lupus, rheumatoid arthritis, ankylosing spondylitis, fibromyalgia and Ménière's disease. I also have asthma-related conditions.… I am one of the 50 million Americans who live with autoimmune diseases, not just one disease, but several."