Miasma: A poisonous vapor or mist believed to be made up of particles from decomposing material that could cause disease and could be identified by its foul smell.
The miasma theory of disease originated in the Middle Ages and persisted for centuries. During the Great Plague of 1665, doctors wore masks filled with sweet-smelling flowers to keep out the poisonous miasmas. Because of the miasmas, they sanitized some buildings, required that night soil be removed from public proximity and had swamps drained to get rid of the bad smells.
However, the miasmic approach only worked if something smelled bad. In the winter, sanitation was forgotten. The theory of miasmas was still popular in the 1800s and led to the "Bad Air theory" which lasted until the 1860s and 1870s. Miasmic reasoning prevented many doctors from adopting new practices like washing their hands between patients. Lethal agents traveled by air, they thought, not lodged beneath a doctor's fingernail.
Although the miasma theory proved incorrect, it represented some recognition of the relation between dirtiness and disease. It encouraged cleanliness and paved the way for public health reform. The pioneer nurse Florence Nightingale (1820-1910) firmly believed in miasmas and became celebrated for her work in making hospitals clean, fresh and airy.
The miasma theory also helped interest scientists in decaying matter and led eventually to the identification of microbes as agents of infectious disease.