Mark and Julie spolier...
What do you supposed the illness was described as "insanity"...? Bipolar, borderline personality disorder, schizophrenia..?
...hope is a good thing, maybe the best of things, and no good thing ever dies.share
A little late for a reply, but I'd suggest some form of psychosis, probably schizophrenia. Schizophrenia is known to run in families; it's believed to be hereditary.
I'm pretty sure when this film was made 'insanity' was just a label they gave to anyone who exhibited any form of psychosis.