Germ theory, proposed by Luis Pasteur, is the idea that disease and infection is caused by microorganisms. Although germ theory had been proposed before it was never believed or accepted. It wasn't until Luis Pasteur developed pasteurization that it was more widely accepted. In 1859, Pasteur disproved spontaneous generation. Spontaneous generation is the idea that organisms develop from nonliving matter. In order to do this he conducted an experiment by boiling broth to sterilize it then leaving one flask of broth exposed to the open air, the other plugged with cotton, and the other flask had a long neck which prevented bacteria from reaching the broth. Over time the flask that was exposed to open air became contaminated while the other two remained sterile. Pasteur proved that spoiled milk and the production of wine from grape juice was caused by microorganisms. This proved the existence of germs. 

Richardson, Robert G., and Douglas James Guthrie. Verification of the Germ Theory. 27 Aug. 2020, www.britannica.com/science/history-of-medicine/Verification-of-the-germ-....