Lycanthropy

Lycanthropy: A particularly cleverly built family of curses, a Lycanthropic curse lies dormant most of the time, sapping the soul's strength and simply storing the energy. Then when the triggering condition occurs, releasing that energy for the designated effect.

That said, there are many flavors of Lycanthropy, covering the trigger (full moon, night, and new moon are common, but some varieties are tied to the victim's emotional state), the effect (generally a full or partial transformation, and different inspiring animals), and the transmission condition (a bite being the most common, but in some instances it requires an elaborate ritual, and occasionally a tiny scratch is all that's needed).

Some few individuals have control over the curse, but its not clear if this is a matter of the individual overcoming the curse, a copy error, or even the original intent of the curse to work this way.