Whilst I don't have a rare disease, I do have a very rare form of Epilepsy, where I become incredibly paranoid and violent, something which only around a dozen other people in the UK have been diagnosed with.
I've had other forms of Epilepsy for much longer, so it was obvious it was the Epilepsy, though I still had my Doctors re-check it and have a full set of tests done to make sure there were no overly damaging repurcussions.