PROBLEMS OF PATHOGENIC MICRO-ORGANISMS 231 



tubercle bacillus, though of great virulence, does not possess the 

 specific power of entering the human body, or, it may be, of main- 

 taining itself after entry in certain tissues, such as the lymph-nodes, 

 except under certain accidental, favoring conditions not yet under- 

 stood. Perhaps the process of cultivating vaccine virus in the skin has 

 deprived it of the capacity for entering through the respiratory tract, 

 and has converted it into a purely inoculable virus. 



In the study of pathogenic bacteria the relative ease with which 

 pure cultures may be obtained from the blood and other organs only 

 accessible by way of the blood has made this a favorite way of 

 obtaining such cultures. But it may be asked whether we are not in 

 this way obtaining bacteria of maximum virulence. May not the 

 non-agglutinability of some typhoid bacilli immediately after isola- 

 tion be accounted for in this way? In general, the bacteria thus 

 obtained can differ but little from the type, as they are all recently 

 descended from a single bacillus, or a very few which caused the infec- 

 tion. It is different in the so-called passages through series of animals 

 in which the usual portals of entry and exit are circumvented and 

 the bacteria injected into the body and withdrawn therefrom directly. 

 As a result of such passages many species of bacteria have been made 

 more virulent, and Pasteur was able to modify greatly the unknown 

 virus of rabies. 



Besides the maintenance of virulence, and its occasional augmenta- 

 tion, a slow decline to complete loss of virulence may be looked for 

 under conditions abnormal for the microbe. This probably goes on 

 where the bacteria multiply, partly or wholly protected from bac- 

 tericidal influences. The bacilli of tuberculosis, which multiply in 

 cavities of the lungs or in muco-pus of the air-tubes in chronic cases, 

 must be regarded as degenerating in virulence. And we actually en- 

 counter races varying considerably in pathogenic power. In the 

 throats of well persons, or those who had diphtheria months ago, 

 bacilli without any power of toxin production, but with all the other 

 characters of genuine diphtheria bacilli, are occasionally encountered. 



During the elimination of the more virulent races of micro-organ- 

 isms, there goes on as well a gradual weeding-out of the most suscept- 

 ible hosts. In a state of nature in which medical science plays no part, 

 there must occur a slight rise in the resistance of individuals, due to 

 selection, and perhaps acquired immunity, which meets the decline 

 of virulence on the part of microbes until a certain norm or equilibrium 

 between the two has been established. This equilibrium is different 

 for every different species of micro-organism, and is disturbed by any 

 changes affecting the condition of the host or the means of trans- 

 mission of the parasite. One result of the operation of this law is the 

 low mortality of endemic as compared with epidemic diseases. Cer- 

 tain animal diseases, while confined to the enzootic territory, cause 



