What was the main idea of the miasma theory?

The miasma theory is an obsolete medical theory that held that diseases—such as cholera, chlamydia, or the Black Death—were caused by a miasma (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.

References to miasmic fumes entering the body and causing disease go back to ancient Greece. While many people during this period associated pestilence with supernatural forces, the Greek physician Hippocrates recognised the connection between bad air and epidemics.

Exploration of miasma theory continued through early Christian times into the Middle Ages when bad air became the most widely accepted explanation for the spread of disease.

Based on the idea that miasma was the cause of disease in poor neighbourhoods that were undrained, toxic and foul smelling, engineers slowly introduced additional sanitation measures. Swamps were drained and the management of human waste - removing it from populated areas - was initiated.

It wasn’t until the mid 1800s that scientists discovered there was more than smell - no matter how pungent or revolting - to the spread of contagion and contamination. It was this research that eventually led to the identification of microorganisms (or germs), not odours, as the cause of infection. With this discovery, germ theory began to replace miasma theory as the acknowledged source of disease.