Severe mental illness, such as full-on schizophrenia, is terrifying. I've seen it tear through two people in my circle, and it seems almost like some sort of divine curse. Your brain just spews pure nonsense at you, and there's nothing you can consciously do about it.

A friend in grad school described his first schizophrenic break: he thought he was a tree, had roots connecting him to the earth, and his role in life was swaying and soaking up the sun. This was someone who was doing a physics doctorate...who occasionally thought he was a tree.

(He was medicated and seemed to have it mostly under control when I knew him.)
But still...the thought your very being falls prey to deranged thoughts you can't control. And it arrives one day in adulthood with no warning.

In Naser's A Beautiful Mind' about brilliant academic John Nash who succumbed to schizophrenia, she quotes him: I'm often asked how such a smart person could believe such nonsense...and the same voice that gave me my brilliant ideas told me all the wild stuff, so I listened.