Our Gemara on Amud Beis describes various odd behaviors that could be indicators of insanity. The Gemara discusses someone who wanders alone at night as not conclusive evidence of insanity, because perhaps “gandripas” took hold of him. What is this problem called גנדריפס?
Rashi here offers two opinions:
- An illness that comes from worry.
- Being overheated and needing fresh air
What is the etymology of this word? Looking at Jastrow, he draws attention to a similar word used in Gemara Yerushalmi (Gittin 7:1) which also discusses forms of mental illness. There the Gemara uses the term Kuntropis. This leads Jastrow to suggest that both words are corruptions of the Greek kunanthropos (κυνάνθροπος) or Lucanthropos (λυκάνθρωπος), which is actually Lycanthropy, a delusional belief that one is a werewolf. (Lycos = wolf, and anthropos = man.)
While etymologically this might be true, it’s doubtful the Gemara would think that someone who is delusional and thinks he is a wolf would still be considered sane. However, there is a possibility that this is so. There are certain regions of the brain responsible for self perception, and on occasion when there is damage to these regions it can lead to people perceiving themselves as having long fangs or claws. The mind then compensates, as with ordinary optical illusions, and fills in the blanks.
It’s an important distinction because unlike psychotic delusions, this might be a limited and local impairment due to some kind of organic brain damage or a tumor. Therefore, indeed it may not be indicative of mental illness and reality distortion globally. So it might just be lycanthropy. Because our sages have traditions going back thousands of years, sometimes even rare medical conditions are noted and discovered via their halakhic ramifications centuries before science developed its own objective tools of observation.
Source
https://www.livescience.com/44875-werewolves-in-psychiatry.html
Translations Courtesy of Sefaria, except when, sometimes, I disagree with the translation 
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