Throw my 2cents in....
Firstly, I believe CarniFreak is right and I'm glad he's posting here. Organic 'drugs' usually involve some chemistry that's way over my head. I've gotten into it to some extent trying to do LSA extractions...but I haven't been messing around for now because it was too much like alchemy instead of chemistry
Anyway, where this thread started I think was very interesting. I have a friend who hallucinates on alcohol, and he's described it to me enough that I do believe him. He is simply predisposed to it, and its liable to happen with sleep deprivation as well. All the same, this example I think just demonstrates the way things work, though less overtly, in all of us. I think the comment about the leaves blowing across the road being recognized as an animal is a good example. We are really talking about a natural phenomenom that certian drugs can play into, but don't necessarilly cause.
Partially, its suggestibility. When you are driving for instance, you are waiting/scanning for an animal, not leaves. So when you notice movement, it enters your awareness as 'animal' to be declassified into 'leaves'. Same sort of thing when you are laying in bed in a dark room, and say there is a chair nearby with a jacket on it. I don't think that most hallucinations or altered perceptions are altogether different phenomena. Generally you first notice visual effects on something that is already strongly patterned, and those effects are in some sense a legitamate interpretation. A lot of times, someone that is tripping will observe things in an exagerrated way (i.e. 'the light through the window is covering the room in a rainbow and smooth and waving'), but someone listening and trying intently to follow these perceptions might be able to notice 'ahh, yes the light coming through that window is in fact refracting and being bent in a wavy pattern as the glass is old enough to have begun to run'.
Nonetheless, I can't put forth with certainty that ALL hallucinations/visuals are created like this, exagerations of some kind of optical illusion or legitamate yet altogether subtle perception. When strongly under the influence of certain psychedelics, I think a person is essential percieving things on a different scale, and can easilly loose the 'big picture' (as in, understanding that they are in a room in a house, which has walls and windows). At this point, the frame of reference is lost, so the imagined gains just as much relevance as the percieved we were talking about earlier.
This is were I think we get into a true hallucination, when your mind has taken information from consensual reality, blown it up to all possible degrees of magnitude, seen it in so exaggerated a facet that it can be morphed into something else, without you having any idea if those changes pertain to a reality. Then your mind can really run with it.
Wild: I've been there with the hypochondria, still am. In some sense, I think you shouldn't try to fight that paranoia, it means something, I think its something you can explore and overcome. I think that it is good to let the experience have an effect on you, and to examine that effect, perhaps learn to use it. If that sounds crazy then, well don't listen to crazy people!