“Oh, your fantasy takes place in the real world, but with magic creatures? You must have to do no world building at all!”
Oh, no world building, but you do have to entirely reevaluate the world’s entire history and think of how modern laws and social conventions would treat the magical creatures in question. Really, it’s worse than world building because you can’t just make shit up most of the time.
Both conditions (in my book’s continuity) are caused by the blood contamination of the uninfected party by the infected party, just like many STDs. It’s a pressing concern in my book, and is the target of medical research and public awareness campaigns like HIV/AIDS in the real world.