They believed the disease was airborne and was spread by miasma, which is a noxious form of "bad air."
To battle this perceived threat, the long beak was packed with such sweet smells as dried flowers, herbs and spices. Also the bird-beak mask provided the doctors with creepy award-winning costumes for any fancy dress party they might be attending.
The doctors' attempt at safety backfired in the sense that the bird masks became a symbol of the Black Death.
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