Ooh, that other poster might be onto something. Either as a permanent or short term super aphrodisiac effect. Supernatural pheromones, accidental exposure to their (pick a fluid), or maybe some kinda alternate procreation to bolster their numbers. There's a lot of ways that could go. Kinda reminds me of the Dresden Files red court vampires saliva, it has a numbing narcotic effect to subdue prey and make them want more