The Germ Theory of disease, which states that illness is caused by germlike substances, was first suggested in the fourth century B.C. by the Greek philosopher Democritus. More than two millennia later, in the latterhalf of the nineteenth century, the French chemist and microbiologist Louis Pasteur and the German physician Robert Koch were finally ableto establish the theory, based on microscopic observation and experimental evidence. For most of the intervening centuries, the concept of Germ Theory wasdoubted and other ideas about the origin of disease prevailed. Predominant among these was the notion that illness was caused by lethal emanations in theatmosphere, also known as "miasmas." Miasmas might be generated from swamps,from the decomposition of plant and animal matter, from sewage, or from various climatic events. As late as the 1840s, the mainstream medical professionin England was espousing the view that poisonous vapors released during decomposition would mix with the surrounding air, penetrate the air cells of the lungs, and spread diseases as varied as cholera, typhoid fever, and plague throughout the human body.
Then, in the 1860s, Louis Pasteur was commissioned by the French government to study the cause of a disease that had been ravishing silkworm populations and threatening the economically powerful silk industry. By 1868 he had determined that a tiny parasite was responsible, and he recommended that to curb its spread the infected silkworms had to be destroyed. Once the silkworms weredestroyed the source of the parasite was eliminated and the disease was halted. Pasteur's demonstration of a link between the silkworm disease and a specific pathogen or disease-causing substance provided the first clear evidence in support of Germ Theory. Pasteur went on to develop techniques for reducingthe virulence of disease-causing organisms, most notably developing the sanitation technique we know today as pasteurization.
Meanwhile, in the 1870s Robert Koch identified the bacterium responsible foranthrax, a disease of cattle. He developed a method for growing the bacteria"in culture," that is, outside of the diseased animal. Placing a drop of hisanthrax culture under the microscope, he observed that as the bacteria multiplied they produced spores. While the bacteria themselves were short-lived, the spores seemed to be resilient. Koch then injected healthy cattle with thesespores and saw that they developed anthrax. This was the first demonstrationthat a disease could be caused by a pathogen that grew outside of a living organism. In 1882 Koch announced the discovery of the bacterium responsible for tuberculosis. In 1883 he isolated and identified the bacterium that causedcholera. Koch developed a four-step method, known today as Koch's postulates,for demonstrating that a particular pathogen is the cause of a disease. First, the pathogen must be found in every individual who has the disease. Second, the pathogen must be isolated from a diseased individual and grown separately in a pure culture. Third, the disease must be induced in an experimental animal by transferring the pathogen from the pure culture. And fourth, the same pathogen must be isolated from the experimental animal after it has contracted the disease.
While Pasteur and Koch demonstrated the connection between specific microorganisms and the occurrence of particular diseases, they did not attempt to explain all disease through Germ Theory. Although we know today that half of allhuman diseases are caused by bacteria, illness can be attributed to many other sources, including genetic mutation, environmental contaminants, malnutrition, prions, and a host of viruses, fungi, protozoans, and parasitic worms.