I think mental illness can be one of the most difficult things to diagnose and treat.
This hits somewhat close to home for me as I have a sister who was diagnosed with schizophrenia in her 50's. Hers is an interesting situation because there is genetic family history of it.
She was involved in a serious car accident when she was a teenager, which she did suffer some brain injury. The doctors believe this is the cause of her problems.
I do not disagree with them, but she is also married to a very controlling man and I think this also play a part in her problems.
I don't know how this fits into the diathesis stress theory, but think there are a number of environmental factors that play a role here.
There could also be some genetic factors that are involved as well, but up to this point, these have not been the apparent cause.