I will probably regret this, given the environment I'm in, but let me use myself as an example. I function in life very well. I am not plagued by destructive thoughts or "voices" and my life is totally normal. However there are times when I choose to participate in a spiritual service, during which I often experience phenomena which could be labelled "supernatural".
People can employ various techniques that involve controlling their breathing, heart rate, and so forth - sometimes with the aid of drugs but not necessarily - and an altered state of consciousness can result. (I am not convined that such states are necessarily healthy, mainly due to things like hypoxia rather than any risk of scary 'demons'.) I don't go in for that stuff. But I have a close friend who uses meditation a lot - crystals, power of the universe, that kind of thing, but not the drugs. She has described some of the experiences she's had while meditating and some are quite interesting. I don't believe that spirits are imparting things to her during these meditation sessions. I believe it fits into the category of altered brain chemistry due to deliberate actions, which sets it apart from mental illness. Just as the physiological state of being drunk is not a mental illness, neither are such meditative states.