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 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.