miasma

Miasma is a historical medical theory that disease is caused by a noxious form of bad air, emanating from decomposing matter, filth, or polluted environments. The term derives from the Greek miasma, meaning pollution or stain. In ancient and premodern medicine, miasma was invoked to explain a range of illnesses and was linked to places deemed morally or physically corrupt, such as swamps, graves, or crowded urban alleys.

Historically, miasma theory influenced public health measures aimed at controlling epidemics. Cities invested in drainage, sewage disposal, sanitation, street cleaning, and better ventilation to remove foul air. Diseases such as cholera, bubonic plague, and malaria were commonly attributed to miasmas, with disfavored air from marshes or markets blamed for outbreaks. The phrase malaria, from the Italian mala aria (bad air), reflected this belief even as the actual transmission of the disease by Anopheles mosquitoes was already suspected in some regions.

The rise of germ theory in the 19th century gradually displaced miasma theory. Pioneering work by John Snow, Louis Pasteur, and Robert Koch demonstrated that specific pathogens—bacteria and viruses—caused disease, shifting emphasis from ambient air to infectious agents. Nevertheless, elements of miasmatic thinking persisted in popular language and some public health practices.

Today, miasma is largely obsolete as a scientific explanation but remains a literary and historical term. It is used metaphorically to describe a pervasive, oppressive atmosphere or an unseen, pollutant hazard, and appears in cultural and fictional contexts to evoke an aura of corruption or danger.