59K views · 721 reactions | In the late 19th century, many thought bacteria were mere byproducts of illness until German physician Robert Koch proved the germ theory of disease. By isolating a microbe, infecting an animal, and re-isolating it, he not only established the link between microbes and disease but also silenced critics and revolutionized medicine. Origins: The Journey of Humankind is streaming on Disney+. | National Geographic Science | Facebook