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Plague Doctors are curious entities that roam in dark areas, or areas that allow natural entities.
In Level 172, a single plague doctor owns a shop which contains basic items sold.
Within Level 205, the first room contains a statue of a Plague Doctor
A mask is sold that turns you into this said entity, that being the Plague Doctor Mask which is purchased at the Mask Shop.
They are neutral entities, but they have similar attack behavior to those of Shadow Ladies. If they are attacked, they could attack you and become hostile.
They wear a long robe with a hood covering their head, which is hidden by a bird's mask. They mostly hold lanterns, and also hold a book of unknown symbols in their glove covered hands.
Be passive around them, play it safe.
Attack them with any weapon.