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.
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