What was the miasma theory?

Miasma theory (also miasmatic theory) was the idea that diseases were caused by bad air (or night air).

Some diseases supposedly caused by miasma were cholera and the Black Death.

"Miasma" comes from Greek roots, meaning "pollution". Hippocrates (4th century BCE) was a proponent.

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.

https://wiki2.org/en/Miasma_theory+Newton

Fortunately for us, the germ theory of disease replaced miasma theory.