Before germ theory, people believed that diseases came from bad air. If you lived in a city in the 1800’s this might have been understandable: Raw sewage, animal processing plants, every sort of chemical use in manufacturing dumped in the nearest river. If you lived in a city, it probably smelled bad! The smell that came off of a river or swamp was thought to bring with it a miasma—an evil stench that caused disease. People either tried to escape the city for the countryside where the air was clean or used perfumes and masks to hide the smell. Today we avoid viruses we cannot see or smell—but the evil is still in the air!
See and read more at: https://www.atlasobscura.com/articles/public-health-bad-smells-miasma