Miasma theory was the belief that it was the smells themselves that caused the disease, or rather, that bad air which was synonymous with with bad smell, that c

Miasma theory was the belief that it was the smells themselves that caused the disease, or rather, that bad air which was synonymous with with bad smell, that caused disease. The plague doctor outfits, for example, were not actually airtight. The 'beak' contained incenses and perfumes intended to protect against miasma by preventing smelling of bad smells. It actually was not very effective (fleas getting in through under the coat was among the many defects,) and the mortality rates of plague doctors was very high.