Since Finder might be investigating him at the moment, let's take a look at the abomination known as the Plague Doctor.
Yeah, he's a quack.
So: the Plague Doctor. Looks like a, well, plague doctor. Robe, stick, beak-nosed mask. There are some reports that the mask isn't, well, a mask at all.
This guy doesn't show up that often -- that's not to say he isn't creepy as fuck, but you most often won't see him before he gives you the fucking Black Death. Yeah, he spreads disease. That's one reason why he's so fucking creepy: even once he leaves, you'll still die from whatever horrible fucking sickness he has given you.
And even if you do see him coming, you may not even realize it. He likes to show up at places where his appearance won't be noticed. Costume parties, carnival, Mardi Gras. Then he goes Masque of the Red Death on people.
I've never seen the Plague Doctor, but I have what he's done to certain Runners. When I tell you that you don't want to know what happened to them, you should believe me.