The Plague Doctor doesn't like to announce his presence, but oftentimes after he leaves a place, someone will put up this symbol:
Looks like some sort of mathematical symbol, right? Well, nope, it's called the Stick and the Serpent. Sort of similar to the rod of Asclepius, except where the rod of Asclepius represents medecine and healing, this symbol represents the fucking Plague Doctor.
Nobody knows who, exactly, puts up this symbol. It just seems to appear, chalk-drawn on the sidewalk or spray-painted on a wall. If you see it, go the other fucking way.