Illustration of a plague doctor (Wikipedia). Plague doctors were physicians hired by towns to attend victims of the bubonic plague. The distinctive costume was the first hazmat suit.
Halloween is all about dressing up as the things that scare us. So, if it’s okay to dress up as the Grim Reaper or a zombie, is it okay to dress up as the latest virus outbreak du jour? National Public Radio has an interesting discussion about the ethics of dressing up as an Ebola health care worker here.