Lycanthropia

"For a very long time Lycanthropia, also known as the wereworlf curse, has been considered a manifestation of evil.
It is now widely known that it is the result of an uncurable and contagious infection affecting the brain cells. While in a normal human being the symptoms are limited to that of a regular encephalitis, in those gifted with magic abilities the consequences can be much more grave. A shape-shifter mage would loose control over preserving one's physical enveloppe and take form of a disfugured mound of flesh, with bones, teeth and claws bloated to unnatural proportions, often mistaken for giant wolf. This hideous figure would crave to absorbe more human flesh in order to restore it's original form, but to no avail, and infecting those unlucky ones who would survive the encounter.
While in teh day-to-day life potions and medicine would help the infected mage keep control over the sickness, they can no longer overcome its effects during the full moon."