40 OFFENSIVE FORCES OF INVADING MICROORGANISM 



organisms is not only increased, but plague pneumonia invariably 

 develops on infecting other rats even by the subcutaneous or intra- 

 peritoneal route. 



Virulence of this order which is specifically directed against cer- 

 tain organs is spoken of as organ virulence and is evidently destined 

 to play an important role in the future study of infection. Capsule 

 formation, however, can scarcely have anything to do with this 

 phenomenon in itself, while we can well imagine that nutritional 

 factors may play an important role. We can thus conceive that 

 during the primary infection, which in turn may be possible owing to 

 capsule formation, a certain group of organisms may have become 

 lodged in a certain organ, and that their vegetative functions here 

 become so modified that the particular juices which are there avail- 

 able can be utilized especially well. If, then, members of this strain 

 are subsequently introduced into another animal, those will develop 

 with special readiness which are placed in contact with the same 

 nutriment to which they had become accustomed in the first host, 

 while the remainder, from lack of this special nutriment, may not 

 develop at all. As a consequence that organ will become the special 

 seat of infection and disease in which conditions for the growth of 

 the organism are most favorable. The affinity for such an organ, 

 may of course be a natural one, and exist already on the part of an 

 organism which has not been passed through an animal for many 

 generations, but there can be no doubt that it may also be 

 acquired. 



Attenuation. The influence of animal passage upon the aggres- 

 sivity of an organism can thus be twofold i. e., it may lead to cap- 

 sule formation, on the one hand, and to a general increase in its 

 functional efficacy as a consequence of especially favorable nutri- 

 tional conditions, on the other, the outcome being an increased 

 virulence for the infected animal. The reverse will be caused by 

 those agencies which prevent the development of these aggressive 

 forces. We have already pointed out that the ability to form cap- 

 sules disappears when an organism is grown on ordinary media, 

 and we know that this inability may become permanent; this in 

 itself does not interfere with the viability of the organism as a 

 saprophyte, to be sure, but makes its parasitic existence in the 

 animal body an impossibility. Such a decrease in the virulence of 

 an organism can be brought about in many other ways, although it 



