Miasmas are poisonous emanations, from putrefying carcasses, rotting vegetation or molds, and invisible dust particles inside dwellings. They were once believed to enter the body and cause disease. This belief dates at least from classical Greece in the fourth or fifth century B.C.E., and it persisted, alongside other theories and models for disease causation, until the middle of the nineteenth century. To some extent the belief still persists today. It was most clearly enunciated by Italian physician Giovanni Lancisi (1654–1720) in De noxiis paludum effluviss (Of the poisonous effleuvia of malaria, 1717).
The miasma theory was advanced to explain many important diseases, including tuberculosis and malaria (from mala aria, meaning "bad air"). Many eminent leaders of medical opinion were convinced that the cholera epidemics of the nineteenth century were caused by miasmas, even as the evidence mounted for the germ theory, which was gathering momentum at that time. William Farr stated firmly in his annual report on vital statistics in Great Britain in 1852 that the inverse association of cholera mortality with elevation above sea level confirmed the miasma theory as its cause. Farr rejected John Snow's argument, based on evidence and logical reasoning, that cholera was caused by contaminated drinking water. Though the miasma theory soon proved to be a wrong explanation for the cause of cholera, it was partially sustained as an explanation for malaria. Some malaria control measures based on miasma theory, particularly draining swamps and marshes, are part of modern control strategies; though not, of course, because miasmas cause malaria, but because mosquito breeding sites are eliminated.
JOHN M. LAST