Diseases like cholera, plague, and yellow fever are caused by 'miasma' , noxious vapours or 'bad air' that carries disease directly to the body.
The miasma theory was overturned by germ theory, particularly after John Snow's 1854 cholera investigation traced an outbreak to contaminated water, not air. Pasteur and Koch's work on microorganisms showed that specific bacteria and viruses cause infectious diseases, not toxic vapours. This shift transformed medicine from focus on air quality to identifying and eliminating pathogens.