Here is an article from a well known researcher at Johns Hopkins explaining what is known about
AI diseases. There is a genetic factor involved but most are NOT true genetic diseases. For instance, a rare myositis disease called sporadic inclucion body myositis is not but there is an even rarer form that is, hIBM.
http://www.aarda.org/common_thread_art.html
AI diseases do tend to run in families and that has been statistically documented but not necessarily the same disease. The article explains the role of heredity in most AI disease as well as how triggers work.
Another article that puts things into perspective.
http://www.usatoday.com/news/health/2007-09-03-autoimmune-advances_N.htm
Bill