The miasma theory (also called the miasmatic theory) is an obsolete medical theory that held that diseases—such as cholera, chlamydia, or the Black Death—were caused by a miasma (μίασμα, Ancient Greek for "pollution"), a noxious form of "bad air", also known as night air. The theory held that epidemics were caused by miasma, emanating from rotting organic matter. Though miasma theory is typically associated with the spread of contagious diseases, some academics in the early nineteenth century suggested that the theory extended to other conditions as well, e.g. one could become obese by inhaling the odor of food. The miasma theory was advanced by Hippocrates in the fourth century B.C. and accepted from ancient times in Europe and China. The theory was eventually abandoned by scientists and physicians after 1880, replaced by the germ theory of disease: specific germs, not miasma, caused specific diseases. However, cultural beliefs about getting rid of odor made the clean-up of waste a high priority for cities.
The word miasma comes from ancient Greek and means "pollution". The idea also gave rise to the name malaria (literally "bad air") through medieval Italian.