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Virchow's discovery of the connection between cancer and cells helped found a new type of medicine. Within a few years, the transformation of medicine into a modern science would be complete with the discovery that bacteria--also cells--were responsible for a wide range of epidemics such as cholera, tuberculosis, and the plague. Two major figures in this revolution were the German physician Robert Koch (1843-1910) and the French scientist Louis Pasteur (1822-95).
The idea that diseases were caused by tiny parasites had a historical precedent. In 36 BC, the Roman scholar Marcus Terrentius Varro had warned people not to build their homes too close to swamps because such areas "breed certain minute creatures which cannot be seen by the eyes, but which float in the air and enter the body through the mouth and nose and cause serious diseases." Arabic physicians of the Middle Ages suggested that microscopic substances were responsible for infectious diseases. But well into the 19th century, most European physicians still held to a miasma theory of disease, which suggested that tiny particles of decomposed material floated through the air, accompanied by unpleasant smells, and caused sickness through poisoning. The idea was useful in a way: It encouraged improvements in sanitation that often resulted in better health. Clean air and water usually held far fewer dangerous microorganisms. Attributing disease to bad air, however, was like a sleight of hand in which a magician gets the public to look at the wrong hand.
The widening use of microscopes began to change this situation. In 1840, Friedrich Henle (1809-85), one of Johannes Moller's assistants in Berlin, wrote "On Miasma and Contagia," an essay in which he challenged the prevailing theory. No one had ever proven that miasma existed, he wrote; it was more likely that the air was merely the route taken by tiny living parasites as they moved from one host to another. Disease organisms had not been found, he said, because they looked so similar to the tissues they infected. While Henle had yet to identify such organisms, he was confident that they would be found, and he joined Moller in pushing medical students to spend time at the microscope.
As this new idea of disease was being introduced in Germany, a researcher in northern France was about to perform an important experiment that would help confirm it. Louis Pasteur was a gifted chemist who had become more and more interested in microorganisms. In the early 1850s, he took on the question of fermentation--the type of chemical transformation that occurred in the production of wine and other types of alcoholic drinks. People had made use of the phenomenon for thousands of years without understanding why it happened. Pasteur proved that this chemical problem was actually a biological one. Within fermenting liquids he discovered yeast cells. He wrote, "I am of the opinion that alcoholic fermentation never occurs without simultaneous organization, development and multiplication of cells."
Where did the cells come from? Virchow's theory of Omnis cellula e cellula was still new. Most researchers were convinced that complex, visible organisms such as maggots came from eggs already found in rotting meat or which had somehow drifted onto it, but the origins of microorganisms were less clear. In 1864, Pasteur published the results of a very careful set of experiments in which he demonstrated that microbes such as bacteria or yeast could not arise in sterile conditions. "Never will the doctrine of spontaneous generation recover from the mortal blow struck by this simple experiment," he wrote. . .
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