In the first century BC, nearly two thousand years before the invention of the microscope, the Roman scholar Marcus Terentius Varro made a remarkable observation about the cause of disease. Writing in his agricultural treatise De Re Rustica around 37 BC, Varro advised landowners to avoid building homes near swamps. His reason was unusual for the time:
There are bred certain minute creatures which cannot be seen by the eyes, which float in the air and enter the body through the mouth and nose and there cause serious diseases.
The statement sounds astonishingly similar to the principles of germ theory. Varro described invisible living organisms too small to be seen, entering the human body and causing illness. Although he lacked any means of observing microorganisms and had no scientific evidence to support his claim, his words represent one of the closest approaches to germ theory found in the ancient world.
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