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in observing techniques, and above all on the invention of methods for 

 handling bacteria and growing them in pure culture. Practically all 

 the early observations were made upon free-living forms as found in 

 nature. Their role in such natural phenomena as fermentation and 

 putrefaction was not understood at all ; most workers, in fact, regarded 

 microbes as the result of these reactions rather than the cause. These 

 ideas implied a belief in the theory of spontaneous generation. Thus, 

 before any real progress could be made in understanding the role of 

 bacteria in the economy of nature, this theory had to be disproved. 

 The story of the battle over abiogenesis is too long to recount at this 

 time. Through the labors of many workers, especially Louis Pasteur 

 and John Tyndall, it was finally shown beyond the shadow of a doubt 

 that bacteria are not generated in fermenting or putrefying materials 

 but in fact are the causes of these reactions. It was demonstrated that 

 if proper precautions were taken to keep microbes out of such things as 

 milk, urine, blood, grape juice, flesh, etc., no fermentation or putrefac- 

 tion occurred. Furthermore, it was demonstrated that specific reac- 

 tions were associated with the presence of specific micro-organisms. 

 Now the way was cleared for an attack on one of the oldest of human 

 problems, the cause and prevention of infectious diseases. From time 

 immemorial men had lived in helpless dread of plagues and epidemics. 

 They were attributed to evil spirits, the wrath of God, or to such 

 assumed natural causes as miasmas or noxious emanations from 

 swampy or low-lying areas, or climatic conditions. Thus the name 

 "malaria" (literally bad air) is a verbal fossil surviving from the days 

 of miasmatic thinking. But from time to time some bold thinkers 

 put forth the notion that invisible living agents might cause infectious 

 diseases. After the discovery of bacteria, the number of these specula- 

 tions increased. But no one came forth with any proof. In 1840, 

 Jacob Henle, a German pathologist, published a small monograph in 

 which he examined this question. He argued that the time was ripe 

 for an experimental attack on the problem of infectious disease and 

 pointed out that there was some very suggestive evidence indicating 

 that microbes might in fact be the causative agents. Henle drew up 

 a set of postulates or principles which would have to be satisfied in 

 such a demonstration. First of all it would have to be shown that a 

 specific organism was invariably associated with a specific disease. 

 Second, it should be possible to separate the specific organism from the 

 diseased body and grow it in pure culture. Third, it would have to 

 be possible to produce the disease in susceptible animals by infecting 

 them with this organism and then reisolating it. Twenty-five years 

 later, Henle's brilliant pupil, Eobert Koch, working in a home labora- 

 tory with homemade equipment, demonstrated the validity of these 

 criteria (hence generally known as Koch's postulates) in anthrax, a 



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