While doing research on an ancestor of mine I came across a transcription of the Georgia Central State Hospital admission records from 1842-1861. It is fascinating (albeit often quite tragic) to flip through, and it has sparked many, many questions about the perceptions and treatment of mental illness in the mid-nineteenth century. One interesting thing that I have noticed is that many of the records list an unrelated disease, such as typhoid or scarlet fever, as the "cause of insanity." How did doctors of this time believe that a virus or bacterial infection altered the psyche of someone who had previously been considered mentally sound? What sort of behavior in patients recovering from an illness would lead to the conclusion that they had become "insane" as a result?
One thing to note is that 19th century medical literature didn't distinguish between cognitive and psychiatric disabilities, so sometimes what they're talking about is someone whose brain was damaged by an illness so that they now have trouble speaking or thinking clearly. Even symptoms of depression or psychosis can be caused by infection or organ failure. This is still the case: many physical illness can cause cognitive and psychiatric symptoms.