A delusion is basically cognitive dissonance caused by irrational thoughts or beliefs. Paranoia (irrational fear) is a type of delusion that I experience while under stress, but so are thoughts you're going to be the next Einstein, which I experience while in a long term hypomanic state. The thing is that you really don't recognize them while they're happening, so you've got to come up with something to get through to you (people, signs, artwork, etc) and redirect you toward something productive and positive. Though paranoia is difficult to direct, so medication is normally an option there.