Hi Jen,

The cause of many autoimmune diseases is unknown, however there may be commonalities in elements of the disease process, including the interleukins seen.  I don't know much about all of the research and science, but I can tell you from personal experience that, yes, people with MS can (and often do) have other autoimmune diseases.

Take me for instance, I have both MS and Rheumatoid Arthritis which is another autoimmune disease. 

Here was a study which looked at - "The occurrence of autoimmune diseases in patients with multiple sclerosis and their families."  Their conclusion "suggests that individuals with MS have a genetic predisposition to autoimmunity in general."

The National MS Society is supporting research into the genetics associated with developing multiple sclerosis.  Here is updated information on genetic research.

Why does it happen?  Well, that is still undergoing scrutiny in the research science field.  If they can figure out WHY, then perhaps it can be prevented.  Now, I've got some more research to do myself (looking for research).

Thank you for your question and I viewed some interesting material while searching for a more concrete answer for you.  Thank you, I learned something.