Samuel Weiss founded germ theory by identifying a transmissible 'material substance' causing infection that could be removed by handwashing.

“and he insisted that the doctors washed their hands with a diluted sort of a a diluted version of bleach and he saw that suddenly now the mortality rate plummeted and so he made this argument now remember he didn't have a microscope he didn't have this is all in the abstract but he made this argument that this material substance was responsible for this what we what was then called child red fever or maternal uh infections and the transfer of the material substance could be removed by washing hands by hand washing and so he in the abstract sense he had basically founded germ Theory yeah I mean completely prescient yeah an incredible aggression and the idea of a material substance that's what's important about it um right it wasn't just bad air in a vague it was on bad air it was not you know uh bad humors it was not it was a material substance and of course if you had if he had the capacity to look down the microscope he would have found out that that material substance was in fact uh nothing else but uh but uh web germs”