Germ theory of disease

The germ theory of disease states that many diseases are caused by
microorganisms, and that microorganisms grow by reproduction, rather than
being spontaneously generated.

Microorganisms were first observed by Anton van Leeuwenhoek.

Louis Pasteur demonstrated that fermentation and the growth of
microorganisms in nutrient broths was not caused by spontaneous generation.
He exposed freshly boiled broths to air in vessels that contained a filter
to stop all particles passing through to the growth medium: and even with no
filter at all, with air being admitted via a long tortuous tube that would
not pass dust particles. Nothing grew in the broths, therefore the living
organisms that grew in such broths came from outside, as spores on dust,
rather than being generated within the broth.

John Snow believed that the germ theory explained outbreaks of cholera, in
opposition to the prevailing miasma theory of cholera.