miasmas

Miasmas, or miasmata, refer to a historical hypothesis that diseases such as cholera, plague, and malaria were caused by vapors or noxious gases exhaled from decomposing organic matter. In this view, foul air—often arising from rotting waste, sewage, or marshes—was thought to be the agent that infected people when inhaled or absorbed through the skin.

The idea has ancient roots in Greek and Roman medicine and persisted through the Middle Ages and into the 19th century. The term mal aria ("bad air") reflects the association of malaria with marshy environments. Miasma theory influenced judgments about urban pollution and sanitation, not only in Europe but in many parts of the world.

Public health measures were often justified on miasmatic grounds: improving drainage, sewer systems, ventilation, and cleanliness to "remove" the bad air. Notable examples include London's mid-19th-century sanitary reforms and the Great Stink of 1858, which helped spur a network of sewers. However, these reforms sometimes operated alongside germ theory, and the two models coexisted for a time in public health thinking.

The rise of germ theory in the late 19th century—linking disease to specific pathogens such as bacteria and viruses—proved more accurate, leading to the decline of miasma theory in modern medicine. Today, the term remains of historical interest and is used metaphorically to describe perceived unhealthy environments or atmospheres.