Why does the body attack itself in autoimmune diseases?

Question

And if it’s possible - How come it doesn’t happen most of the time?

Answer 1

IN Autoimmune disease body thinks its own parts are foreign bodies...so as a natural response its sends its "army" to tackle the prob....the cause of this still unknown...there r different theories but none have been proved as yet....  

Answer 2

I am no expert, but its my understanding that the body has cells that attack "bad" cells. This is your immune system. A problem occurs if the body cannot tell which are the baddies and which are the goodies and it ends up attacking good cells as well.