Germ theory of disease
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The Germ theory of disease is a theory in biology. It says that small organisms (called germs) are the cause of many diseases. Today's view of how disease happens is that those small organisms cause a reaction in the body of those affected by a disease. This reaction is what people call the disease.
Louis Pasteur showed that this theory was true. He made an experiment with fermentation. In one case, he made a broth that he isolated from the sourroundings, so no particles could pass to it from outside. The broth he thusly made did not change, and no fermentation took place.
The other case was a similar broth, but this time, it was connected to the environment. This time, fermentation took place. The thing that caused fermentation therefore needed to come from outside (the environment).
Another view of how diseases came to be was that they sponaneously generated. Scientists call that view abiogenesis, with the meaning of life being generated from non-living forms.