All Autoimmune Disorders have two factors in common

"1. A hereditary predisposition thought to be a combination of certain (as yet mostly unknown) genes.
2. An environmental trigger or triggers. Again many are unknown. It often occurs after recovery from infection. Some bacteria produce toxic products that cause T cells to bind to macrophages - perhaps this is how T cells learn to recognize the body's own tissues, which they now attack. (Autoimmune disorders are caused when cytotoxic T cells and normal antibodies attack an individual's own body cells).

In addition, Autoimmune Disorders can be difficult to diagnose. Symptoms may vary from person to person. Remission (where no outward signs of the disorder are present) is common, sometimes for many years.

It is also not unusual for a person with an undiagnosed Autoimmune Disorder to be told they are malingering, a hypochondriac, etc.
A few have even been sent for Psychiatric treatment."

For more information:

http://www.autoimmunity.co.uk/
http://www.autoimmunity.co.uk/information/common.php