SWIM may or may not have confided to me experiences with a variety of compounds purported to induce a wide range of subjective internal experiences upon their various methods of consumption. At any rate, I've certainly read (and donated to) Erowid.
Not OC, and I've never tried psychedelics, but even a strong fever will make you hallucinate, and I've had a couple of those. You mind closes up into itself, and the world it creates, while extremely simplistic, feels very real.