There's a rare neuropsychiatric condition called Cotard's Syndrome where a person holds the delusional belief that they are dead or decaying, have lost their internal organs or blood, and even more rarely, can include delusions of immortality.
This is like the opposite of Sixth Sense syndrome, where a dead person thinks they're really alive (though who knows if that happens, I'd love to find out, but not the hard way).
So it seems that the people who suffer from this feel like they're dead (or ghosts?) and that everything around them isn't real. I'm sure that seeing movies like The Matrix doesn't help if someone does lean towards this condition - there are enough people already who are using the Neo defense, saying that they don't believe the world around them is real.
I'm wondering whether the people who think that they're decaying can actually see the decay, whether they have the visual hallucinations to reinforce this, or whether it's just a vague sense that they're decaying from the inside out.
This seems related to body dysmorphia, where healthy people wish to have a limb removed (dramatically exaggerated in Grey's Anatomy when someone takes a chainsaw in the ER that came in with another patient and saws off their own leg).
The brain is a strange beast, and sometimes produces strange results.
