I am afraid that there is no simple answer.
The body's immune system is a hughely complex thing.
A lot of factors, ranging from genetic disposition, exposition to certain toxins (inkl those from certain funghi) to psychological stress may be involved.
Some consider even cancer a autoimmuneproblem, as maligne cells would normally be exterminated.
So, the disease should not be considered under its label of being autoimmune. There is no point comparing them to other diseases.