Plague doctors wore those strange masks primarily to protect themselves from the deadly diseases they were exposed to, particularly the bubonic plague. The masks had a beak-like shape which was filled with aromatic herbs, spices, flowers or vinegar-soaked sponges to filter the air and purify it before it entered the mask. The mask also had glass or crystal eyepieces that provided the doctor with a clear view of his surroundings. The long black robe and gloves were also meant to protect the doctor's whole body from infected material.
