Undoubtedly many alleged cases of demonic possession or poltergeist activity are actually due to mental illness. But how many cases of mental illness are due to the action of some toxic substance on the brain, or due to some unsuspected deficiency?
In a few chilling cases, a descent into madness has been precipitated by some problem that could have easily been avoided, with the correct knowledge.
I thought it would be interesting to write a story exploring this idea: a story featuring an unsuspecting victim who accidentally deprives himself of some important nutrient, and thereby slides into madness, losing friends and relationships and ultimately entangling himself with a dubious self-proclaimed exorcist, all the while gravitating to superstitious explanations of his misfortune.
While researching ideas for this story I looked for known unusual organic causes of psychosis and came across several interesting possibilities.
Vitamin deficiences are in general not known to cause psychosis, but in certain rare cases, it appears that's exactly what happened.
See for instance: https://www.cureus.com/articles/156057-psychosis-and-seizures-attributed-to-severe-vitamin-b12-deficiency-a-case-report
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