In 1876, Robert Koch proved that bacteria could cause disease

Building on the germ theories of Louis Pasteur, the German doctor Robert Koch developed new methodology to test whether a particular micro-organism (Bacillus anthracis) was the cause of anthrax. He extracted the bacterium from sheep that had died of anthrax, grew the bacterium and then injected a mouse with it. The mouse developed the disease. Koch’s team later identified the bacteria that caused tuberculosis in 1882 and cholera in 1883. Other researchers adopted his methods and were able to identify the bacteria that caused typhus (1880), tetanus (1884) and the plague (1894).