The gist of the book is generally: aliens invade earth and the main character gets infected by alien parasites and slowly spirals into insanity across the span of the book.
The element I'm looking for is the "main character gets infected/inhabited by an alien (could also be a ghost, demon, other-worldy creature, etc) and it slowly driving them insane across the span of the story.
Forgive me for specifics, I know there's a lot of "spiralling to insanity" books and I know there's a lot of "alien (ghost/demon) inhabiting main character" books but I'm looking for something that combines the two.
Thanks in advance!
by 9aegerist