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The Germ Theory

The Germ Theory


The Germ Theory. Scientists developed the modern approach to understanding and controlling epidemic diseases during the last quarter of the nineteenth century. In 1862 French scientist Louis Pasteur (1822-1895) showed that airborne bacteria were the cause of fermentation, thus giving rise to the germ theory, which replaced an older theory that attributed diseases to environmental causes. In 1876 German scientist Robert Koch (1843-1910)who was studying anthrax, a disease of sheep and cattledemonstrated that specific diseases were caused by specific pathogens (the agents, such as bacteria or viruses, that cause disease), and in 1879 Pasteur found that he could use the bacilli (rod-shaped bacteria) that caused various diseases to vaccinate people against them, generalizing from the discovery of the smallpox vaccine by British physician Edward Jenner (1749-1823) in the late eighteenth century. During the 1880s and 1890s scientists identified the pathogens responsible for many diseases, including cholera, diphtheria, tetanus, tuberculosis, and typhoid.

Medical Technology. The discoveries leading to modern bacteriology were based on a series of technological innovations. The first was the development in the 1860s and 1870s of aniline stains, dyes that could be applied to cultures of bacteria to make them visible under a microscope. The second was the introduction of microscopes that could yield images of bacteria at high magnification without distortion. All of Kochs discoveries during the 1880s were dependent on the development of oil-immersion lenses.

Vaccines. In 1882 Koch identified the tuberculosis bacillus, and eight years later he developed tuberculin, a vaccination that proved more or less ineffective. Once it was recognized that tuberculosis was a communicable disease, it became possible to adopt public-health methods to combat it, such as disinfecting the homes of the afflicted and isolating them in sanatoriums. Diphtheria represented a more complex problem because its symptoms overlapped with those of other common diseases. One of Kochs assistants isolated the diphtheria bacillus in 1884, but the disease was not fully understood until the next decade. An investigation of 5,611 suspected cases reported in New York City during 1893-1894 revealed that only 60 percent were really diphtheria. By the middle 1890s an antitoxin had been developed, making diphtheria the first of the epidemic diseases to be fully studied and then prevented by a vaccine. As a result the diphtheria death rate for Illinoisa state with highly accurate medical recordsdropped from 113 per 100,000 in 1886 to 22 per 100,000 in 1902. More than any other event, the success of the diphtheria antitoxin is responsible for initiating a phase of bacteriomania in American medicine.


William G. Rothstein, American Physicians in the Nineteenth Century (Baltimore: Johns Hopkins University Press, 1972).

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