My detective is tracking a serial killer - actually a pair of "spree" killers - who turns out to be a lycanthrope.
In the world of my book, lycanthropy is a blood-borne disease that can only be passed blood-to-blood. Being attacked by a lycanthrope won't give you the disease unless the lycanthrope is wounded and bleeds into your wounds. Lycanthropy was very rare in the world until about the time of WWI. With blood transfusions becoming more and more common after that time, it caused a population explosion in lycanthropes. By the time of the story - now - lycanthropes have become so common as to have won legal citizenship. There's a lot of bigotry toward "shifters" and some states allow them to be killed out-of-hand under "varmint" laws.
My protagonist will have to deal with tracking down a serial killer who shares the disease that he is learning to live with. His fellow cops share the public's dislike for lycanthropes. They are, perhaps, more conservative than the general public and more hostile. Even his own partner has problems with lycanthropy. When all the work problems are resolved, he has to tell his best friend, his family, his girlfriend, and a bunch of others that he becomes furry once a month. <sigh>
Lycanthropes (I know I've been over-using the word) come in many varieties. When you contract the disease, you never know what kind of beast you'll become. There are were-wolves, were-rats, were-opossums, were-whatevers. The beast that my protagonist becomes and the beast he must track down will play heavily in the unfolding of the story.
I'm done. Hope to post an excerpt in louderprose in the next few days