The plague doctor costume was designed to protect the wearer from airborne illnesses.
Katharine Trendacosta via io9:
… Read the rest
First up is the wide-brimmed leather hat. As we’ve discussed before, no job in the 17th century really meant anything unless it had an official hat. Lawyers had wigs, the clergy had a whole mess of headgear, doctors had these. Because you know how disease hates hats.
The birdlike mask—which may predate the outfit by several centuries—was a gas mask. Sort of. de Lorme’s description says the beak should be “filled with perfume” and herbs stuffed into it. That’s because of the miasma theory of disease.
Miasma theory posited that disease was carried by a cloud of poisonous vapor in the air, which was created by decay and could be identified by a bad smell. Following the logic that bad smelling air carried disease, it makes perfect sense that you could “cure” the air by making it smell good.