How did scientists prove that germs cause disease?

The germ theory was built in steps. In the 1850s and 1860s Louis Pasteur showed that living microorganisms, not spontaneous generation, were behind fermentation and spoilage. In the 1870s and 1880s Pasteur and Robert Koch showed that particular microbes cause particular diseases. They grew a microbe in pure form, used it to give healthy animals the disease, and then recovered the same microbe from them. Koch's criteria for this method became known as Koch's Postulates. [1] [2]

Microbiology as a distinct science is traditionally dated to 1857, when Pasteur convincingly demonstrated that microorganisms were responsible for the fermentation of fluids. His work undermined the theory of spontaneous generation and showed that spoiling or contamination of organic substances came from microorganisms in the environment. A textbook history says this essentially proved the germ theory of disease and launched modern microbiology. [pmc.ncbi.nlm.nih.gov](https://pmc.ncbi.nlm.nih.gov/articles/PMC7176178/) [1]

In a widely taught experiment, Pasteur boiled broth in swan-neck flasks to kill existing microbes. When the neck of one flask was broken and the broth exposed to the air, it went bad, while the flask whose neck was left intact did not. This supported his view that microbes in the air cause spoilage, and it suggested that they might also cause infection. [bbc.co.uk](https://www.bbc.co.uk/bitesize/guides/z6kt6g8/revision/2) and [bio.libretexts.org](https://bio.libretexts.org/Bookshelves/Microbiology/Introduction_to_Microbiology_(Liu_et_al.)/04%3A_Prokaryotic_Cell/4.03%3A_The_Germ_Theory_of_Disease) [3] [4]

Koch, a district medical officer in Prussia, studied anthrax in the 1870s. He found anthrax bacilli in the blood of infected sheep and passed the infection to healthy experimental animals. He also described the bacterium's life cycle and its formation of endospores, and published this work in 1876. Another account describes him examining the blood of cattle that had died of anthrax, seeing rod-shaped bacteria, and then giving mice the disease with that blood. [pmc.ncbi.nlm.nih.gov](https://pmc.ncbi.nlm.nih.gov/articles/PMC7176178/) and [ncbi.nlm.nih.gov](https://www.ncbi.nlm.nih.gov/books/NBK24649/) [1] [2]

Koch also developed tools that made such proof practical: oil immersion microscopy, staining methods, and pure cultures grown from single colonies on solid agar. In 1882 he isolated the tuberculosis bacterium, Mycobacterium tuberculosis, in pure culture. The criteria he proposed, as refined, are that the pathogen must be found in every case of the disease and not in other diseases as a harmless organism, that it must cause the disease in an animal after repeated growth in pure culture, and that it must be re-isolated from that animal. [pmc.ncbi.nlm.nih.gov](https://pmc.ncbi.nlm.nih.gov/articles/PMC7176178/) [1]

Pasteur reached a similar conclusion by another route. With Joubert, he grew the anthrax bacterium through many successive cultures in sterile fluid, so that the original drop was diluted to almost nothing. The final culture still produced anthrax with all its symptoms in animals, which he called indisputable proof that anthrax is a bacterial disease. He described the same approach as the only way he knew to show that a microorganism is the cause of a disease. His team's work on septicemia was harder: early cultures failed or were contaminated, and they found the septic vibrio grew only without oxygen. [sourcebooks.web.fordham.edu](https://sourcebooks.web.fordham.edu/mod/1878pasteur-germ.asp) [5]

The postulates are described as still valid to some extent, but they do not fit every case. Some microbes cause disease without being easily cultured, and the results here do not cover those cases. One source stresses that animal experiments remain central to proving that a specific germ causes a disease. [pmc.ncbi.nlm.nih.gov](https://pmc.ncbi.nlm.nih.gov/articles/PMC7176178/) and [ncbi.nlm.nih.gov](https://www.ncbi.nlm.nih.gov/books/NBK24649/) [1] [2]