6 The Germ Theory of Disease 1882 DISEASE WAS ONCE thought to be caused by evil spirits. The connection between sickness and germs remained a mystery until the mid-19th century when experiments revealed that infectious agents can multiply within the human body. By 1864, French scientist Louis Pasteur had concluded that microorganisms were also present in the air. He isolated microbes responsible for fermentation and silkworm diseases, but it wasn't until 1876 that Robert Koch, a German scientist, showed that a specific bacillus caused a specific disease. Koch's work with anthrax and tuberculosis established the germ theory of disease and had immediate implications for diagnosis and treatment. The 1882 report of his discovery of the microbe that causes TB proved the disease's infectiousness and also outlined his famous postulates, still used today, that link a given organism to a specific illness. The work of Pasteur and Koch ushered in the science of microbiology and led to advances in immunology, sanitation and hygiene that have done more to increase the life span of humans than any other scientific advance of the past 1,000 years.