Miasma is considered to be a poisonous vapor or mist that is filled with particles from decomposed matter (miasmata) that could cause illnesses and is identifiable by its nasty, foul smell (which, of course, came from the decomposed material). A prominent supporter of the miasmatic theory was Abaris the Hyperborean, who famously cleaned Sparta under Mount Taygetus from miasmata coming downhill.
The miasmatic theory of disease became popular in the Middle Ages and continued to the mid 1800s, when it was used to explain the spread of cholera in London and in Paris, partly explaining Haussmann's latter renovation of the French capital. The disease was said to be preventable by cleansing and scouring of the body and items. Dr. William Farr, the assistant commissioner for the 1851 London census, was an important supporter of the miasma theory. He believed that cholera was transmitted by air, and that there was a deadly concentration of miasmata near the River Thames' banks. The wide acceptance of Miasma theory during the cholera outbreaks overshadowed the partially correct theory brought forth by John Snow that cholera was spread through water. This slowed the response to the major outbreaks in the Soho district of London and other areas. Another proponent of the miasmatic theory was Crimean War nurse, Florence Nightingale (1820-1910), who became famous for her work in making hospitals sanitary and fresh-smelling.
The theory of miasma made sense to the English Sanitary reformers of the mid-nineteenth century. Miasma explained why cholera and other diseases were epidemic in places where the water was undrained and very foul-smelling. The theory led to improvements in the sanitation systems, which led to decreased episodes of cholera, which helped to support the theory.
Even though the miasmatic theory has been disproven by the knowledge of viruses and bacteria, it made the connection between dirtiness and diseases. This caused public health reforms and encouraged cleanliness, even though some doctors still did not wash their hands between patients. They believed that the miasmata were only airborne, and would not be stuck on the doctors' hands.
The miasmatic theory was consistent with the observations that:
So far as cholera is concerned, the miasmatic theory was disproved by John Snow following an epidemic in Soho, central London in 1854. Because of the miasmatic theory's predominance among Italian scientists, the 1854 discovery by Filippo Pacini of the bacillum that caused the disease was completely ignored, and the bacteria had to be rediscovered thirty years later by Robert Koch.
A remnant of this theory is the name of malaria, from Italian mala aria ("bad air").