I have read about damage that can be done to the brain by persistent, long-term depression, although it's not clear to me whether this refers to totally untreated depression or depression that's proven resistant to treatment. In any case, they say at a later stage in life the changes to such a person's neurological functioning can mimic Alzheimer's, which has me fearing that I might well become such a person at some point, even though I'm only 32.