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My understanding is that it is largely hereditary and something in one's life triggers it. In my case I had symptons for many years and they were always diagnosed as something else until a specific doctor recognised it and I was biopsied. I believe a significant stressful time in my life was the trigger. Unbeknown to me, one of my sisters also had it and we had never shared this with each other until quite recently. Genetics is primary and of course it's auto immune then things like food, stress, chemicals etc affect it and bring on flares.