Werewolf, duh.
Off the top of my head... the girls find an old medical storage room with blood samples of an especially crazy person who believed they were a werewolf. Turns out that person wasn't crazy, they
were really a werewolf and the samples in storage are still contagious. They get exposed while goofing around telling the story of the crazy werewolf person that was locked up there. (person's name is on the samples, and one of the group remembers stories about that patient).
How's that sound?