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sults of processes in the system that are nearly analagous to fermentation, and that such diseases are therefore traceable ultimately to the existence of living germs. This view of the case brought Pasteur to a large and general investigation of bacteria. The bacterium may be defined as a microscopic vegetable organism; or it may be called an _animal_ organism; for in the deep-down life of germs there is not much difference between vegetable and animal--perhaps no difference at all. The bacterium is generally a jointed rod-like filament of living matter, and its native world seems to be any putrefying organic substance. Bacteria are the smallest of microscopic organisms. They are widely diffused in the natural world, existing independently and also in a parasitical way, in connection with larger forms of organic life. They multiply with the greatest rapidity. On the whole, the bacterium fulfills its vital offices in two ways, or with two results; first, _fermentation_, and secondly, _disease_. To this field of inquiry Pasteur devoted himself with the greatest assiduity. He began to investigate the diseased tissue of animals, and was rewarded with the discovery of the germs from which the disease had come. It was found that the bacteria of one disease are different from those of another disease, or in a word that the microscopic organisms which produce morbid conditions in animals are differentiated into genera and species and varieties, in the same manner as are the animals, birds and fishes, of the world. A new realm of life invisible save by the aid of the microscope, began to be explored, and practical results began to follow. Pasteur at length announced his ability to _produce_ infectious diseases by inoculation; and of this his proofs and demonstrations, were complete. In the next place he announced his ability to _counteract_ the ravages, of certain classes of diseases (those called zymotic) by inoculating the animal suffering therefrom with what he called an "attenuated" or "domesticated" virus of the given disease. The matter first came to a practical issue by the inoculation of well animals with the attenuated virus. The animals so treated became _immune_; that is, exempt from the infection of the given disease. Pasteur gave public demonstrations in the fields near Paris, using the disease called splenic fever, and sheep as the subjects of his experimentation. The whole civilized world was astonished
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