INFECTION AND DISINFECTION. 771 



The peculiar interest connected with this simple experiment is due 

 to the fact that minute organisms closely resembling those just de- 

 scribed are found in the bodies of patients suffering from acute infec- 

 tious 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 minute- 

 ness of the organisms, necessitating the use of the highest powers of 

 the microscope for their detection. Moreover, as already stated, bac- 

 teria 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 dis- 

 ease, 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 pres- 

 ence 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 organ- 

 isms in fermentation. Guided by these discoveries, Davaine inoculated 

 healthy animals with blood from those diseased, with the result of pro- 

 ducing similar symptoms, while myriads of organisms were found in 



