We all need a partner. Sherlock has his Watson, Robin Hood has his Little John. The Plague Doctor has the Plague Nurse. If you were about to do a medieval battle against infectious miasmas armed with only a lantern, a rudimentary concept of personal hygiene, and a beak full of smelly herbs and flowers, you'd want a second-in-command who's got your back. And your front. And whatever other body parts you're worried about, which is probably all of them.