I agree with franz kafka. Delusions are isolating, in the sense that other people don't share your perception and belief about *whatever*.

Believing in conspiracy theories is not a delusion itself, unless the believer becomes truly paranoid. Believing that Elvis, Tupoc, and Michael Jackson are still alive together somewhere is also not a delusion, unless the believer becomes obsessed with the idea...it's the focus of their every conversation, etc.

Yeah, I'm going with franz k - a delusion is something you perceive, you believe is true, it tends to fill your thinking, you've become paranoid about it, and other people don't "get" what you're believing, or why you're believing it. You feel alone, isolated.