Medically speaking you can't really say what "causes" a hallucination, since hallucinations can occur in any sensory modality. Your perception is based on information from at least nine different senses, all of which can be altered in a plethora of ways. A hallucination is a perception without genuine stimulus, and can be "caused" by anything from a chemical imbalance to an infection, disease or injury, and can even be induced by suggestion and self-hypnosis, psychological trauma, etc. making for purely psychological hallucinations.
A hallucination is when your senses are "tricked" into perceiving something that isn't there.
Anyways! My hallucinations.
1. As a long-term sufferer of sleeping disorders, I frequently experience sleep-deprivation induced hallucinations. I see spiders/insects on the periphery of my vision.
2. Once when I had food poisoning I hallucinated that I was in the future, and as such all electronic devices were sentient and could communicate with me. I talked to the clock for a good period of time. About relationships, no less. I don't actually remember this, but have been told I did it.
3. Once when suffering from sleep deprivation I started to think that the design on the back of my shirt was slowly crawling up. I then came up with the absurd(ly awesome) idea that shirts are actually symbiotic organisms, and that we provide them with transportation in exchange for the chemicals they released into our skin that would reduce feelings of shame and loneliness. I wasn't tired enough to believe the idea really, but I talked about it extensively in a "what if, man!" way.
4. Once when extremely sick I had this hallucination/vivid dream about a God-like being causing the apocalypse. Everyone was trying to escape, piling into trucks and vehicles and driving or running away from "God", in the hopes that there would be a safe zone somewhere. In the end, reality fragmented and shattered, like a mirror. To this day it still disturbs me a little bit because it felt so real. This happened about five years ago, and I actually think it's pretty cool. Goes to demonstrate the mind's power over the body.
5. Another time when extremely sick as a child I hallucinated a white hand breaking through the wall and coming for me (note: it came in from the intersection of the wall and ceiling*). I mostly surpressed it, but afterwards I became convinced I had witnessed something paranormal, that at some point I had actually seen a ghost hand coming for me.
*In some of H.P. Lovecraft's work, demons would enter our dimension through angles, like the intersection of wall and ceiling
