The peculiar interest connected with this simple experiment is due to the fact that minute organisms closely resembling those just described are found in the bodies of patients suffering from acute infectious diseases, and the question naturally arises as to the relation which exists between the organisms and the symptoms. Are the former the cause of the latter, or is their presence a mere coincidence? Another suggestion is that their presence is the result of the disease. If the symptoms are really caused by the presence and action of the bacteria, it would follow that differences must exist between the organisms found in different diseases. Great and manifold difficulties attend such investigations; it is sufficient here to notice the extreme minuteness of the organisms, necessitating the use of the highest powers of the microscope for their detection. Moreover, as already stated, bacteria are found in large numbers in the bodies of healthy persons, and some of these organisms very closely resemble, if indeed they are not identical with, those that have been found in connection with severe infectious diseases. It is hardly conceivable that minute organisms which abound, for example, in the mouth, and give rise to no changes, should be capable in other parts of causing the most serious symptoms.
In order to prove that a micro-organism is the real cause of a disease, at least three conditions must be fulfilled: In the first place, the same species of micro-organism must be invariably found in the parts affected by the disease in question, at any rate during the early stage, and in no other affection. Secondly, the organism must be cultivated apart from the body in which it has been found, so as to make sure that it has been separated from all other morbid materials to the presence of which the disease might possibly be due. Thirdly, when the organisms thus cultivated have been introduced into the body of an animal capable of being attacked by the disease, similar symptoms ought to be set up, and the same micro-organisms should be found in the newly affected animal. If, in testing any given disease, these conditions are fulfilled, it is scarcely possible to doubt that the micro-organisms are the cause; they certainly can not be the result. It is fair also to argue from diseases in which the conditions are fulfilled, that others in which, owing to circumstances, the tests can not be properly carried out, are due to similar causes.
Very strong evidence is forthcoming in support of the theory that micro-organisms are the cause of infectious diseases. Horned cattle and sheep are subject to a disease termed anthrax, or splenic fever, and more than thirty years ago minute, rod-shaped bodies were found in the blood of animals which had died from this disease, which is also communicable to man. The significance of these rods was suspected only after Pasteur's researches into the part played by minute organisms in fermentation. Guided by these discoveries, Davaine inoculated healthy animals with blood from those diseased, with the result of producing similar symptoms, while myriads of organisms were found in
