PROBLEMS OF PATHOGENIC MICRO-ORGANISMS 229 



over the various infectious diseases of man and animals, of which 

 we have any definite information, we shall be surprised to find in 

 how many the points of attack are in organs or tissues in direct 

 communication with the exterior. In the most common type of 

 tuberculosis, pulmonary consumption, the process is almost wholly 

 limited to the respiratory organs. In typhoid fever the process is 

 to a large degree carried on in the intestinal wall. In dysentery and 

 cholera it is wholly so. Even in the very protracted disease of 

 leprosy, the skin is the chief seat of the disease, while the discharge 

 of bacilli from the ulcers of the nose is the rule in the tuberculous 

 type. In that exquisitely parasitic, highly specialized group of 

 micro-organisms producing the eruptive diseases, the final process 

 is carried on in the skin. In these diseases the mechanism of escape 

 is the most perfect. 



On the other hand, among the spore-bearing pathogenic bacteria 

 the means of escape is uncertain. Thus the anthrax bacillus betrays 

 its saprophytic nature, as pointed out by Koch many years ago, 

 in its inability to produce spores within the body. Were it not for 

 the accidental discharges of blood from the mucous surfaces and 

 the operations of man, the bacillus might not escape at all to sporu- 

 late. Similar conditions obtain for the bacillus of tetanus and of 

 Rauschbrand. Both produce disease probably in an accidental 

 manner, and not as progressive parasites. Their continued exist- 

 ence is assured by vegetation and spore formation outside of the 

 body, and it is highly probable that the species would continue to 

 exist if they did not attack animal life, and that their incursions 

 into the bod}'- are of no use to them. On the other hand, all attempts 

 to demonstrate the production of spores in bacteria whose exist- 

 ence depends largely or wholly upon parasitism have thus far 

 failed. The spore is evidently poorly fitted to parasitism, and is 

 replaced by other devices of more adaptability. 



The mechanisms of invasion and escape bear a distinct relation to 

 the pathogenic power or virulence. It is safe to assume that those 

 varieties or species no matter how virulent, will be eventually de- 

 stroyed if these mechanisms are imperfect. In fact, the more virulent 

 the microbe, the more rapid the death as a result of invasion, the 

 less the opportunity for escape. Hence there will be a selection in 

 favor of those varieties which vegetate whence they can escape. 

 The surviving varieties would gradually lose their highly virulent 

 invasive qualities and adapt themselves more particularly to the 

 conditions surrounding invasion and escape. That some such pro- 

 cess of selection has been going on in the past seems the simplest 

 explanation of the relatively low mortality of infectious diseases. 

 These individuals or races of microbes which invaded the host too 

 rapidly and caused death would be destroyed in favor of those 



