From 1867 to 1870, Pasteur studied silkworm diseases affecting France's silk industry. He managed to isolate the pathogens. He established that the two agents responsible for this epidemic were protozoa and bacteria. He began on germ theory as the evidence he collected from his fermentation and silkworm studies proved there were harmful microscopic organisms.

"This disease occurs in epidemics in the silk-raising districts, sometimes to such an extent as almost to annihilate that industry. The caterpillars attacked by it show black spots on their skin, lose their appetite and die. In the blood and all the tissues of the worm small oval granules were observed by the various investigators. Although these granules were recognised as a species of the lowest fungi, their relation to the disease was not established until Pateur proved them to be the cause..."

"...In simple but conclusive experiments he showed that the disease could be produced in healthy specimens by feeding them on mateial containing these granules, or by inoculating them from a wound. On the basis of postive observations he explained thus the spreading of the epidemic. He further showed that the butterfly transmits the disease to its offspring... For the parasitic growth can be found in the egg, if either male or female be tainted with the disease. Although Pasteur did not succeed in finding any means of arresting the disease in the individual animal, he taught the silk raisers how to check the epidemic."

~ Bacteria and the Germ Theory of Disease: Eight Lectures Delivered at the Chicago Medical College