Could werewolves exist in a realistic world?

All that is required for an accepted answer is the ability to change shape over night. Bonus points will be awarded if you can explain a bite spreading transformation and a full moon causing the transformation itself. Could an animal evolve this ability?

A list of all of the Anatomically Correct questions can be found here

http://meta.worldbuilding.stackexchange.com/questions/2797/anatomically-correct-series/2798#2798