Mythos virus

From Wotchipedia

The Mythos Virus is a magical disease that is highly contagious. It will transform anyone who is not already into a mythical creature, ranging from Centaurs to Minotaurs to Hylians. At first it works slowly, taking a few hours to take effect, but as it spreads, the effects are quicker and quicker. Also, it eventually brings with it the instincts of the creatures, such as a Merperson's urge to swim, or a Minotaur's calling to guard labyrinths and block passages. It usually resides in old spellbooks, being transmitted through the book's dust.