Werewolfism entered medieval medicine as folkloric lore recycled from ancient Greece

Evidence

For example, Avicenna, following his predecessors, declared that madness was caused by an imbalance in the biles. In particular, a predominance of black bile was the cause of melancholia (though Avicenna accepted that jinn (demons) could also cause melancholia). Excessive hairiness was one of the symptoms of this sad affliction. Lycanthropia, or werewolfism, was another possible version of melancholia. It is unlikely that Avicenna ever had to treat a werewolf; rather he was unthinkingly transmitting a piece of ancient Greek folklore that had found its way into the medical textbooks. — Robert Irwin, For Lust of Knowing, 70

Categorization

• FICTION: The image of a respected physician gravely diagnosing werewolfism as a bile imbalance is a ready comic or satirical scene.
• CANDY: Fun fact: respected medieval physicians officially diagnosed werewolves via black-bile imbalance, symptomized by excessive hairiness.