PASSIVE AGGRESSIVITY 33 



justifiably so the more evenly the toxic and the infectious properties 

 are represented in the same organism. By a particularly virulent 

 infection we would here mean an infection, during which there is, 

 on the one hand, an active multiplication, and on the other a 

 correspondingly active toxin formation with the production of 

 a correspondingly severe clinical picture. 



Difference in Aggressivity. Having thus established the proper 

 meaning of the term virulence we may now return to the question : 

 To what factor is the difference in the aggressivity and hence the 

 virulence of the different groups or strains of bacteria due? Two 

 possibilities naturally suggest themselves, which may be operative 

 either individually or conjointly. On the one hand we may imagine, 

 that an organism, when introduced into the body of an animal 

 which seeks to destroy the invader, adjusts itself to its new surround- 

 ings by certain changes of a morphological or physiological character, 

 in consequence of which it becomes relatively or absolutely unassail- 

 able by the offensive forces of the host, unless, indeed, it already 

 possesses such properties during its saprophytic existence outside 

 of the body. On the other hand we can conceive that the infecting 

 organism actively secretes material which tends to counteract or 

 even to destroy the opposing forces of the host. 



Aggressins. Such substances Bail has termed aggressins, and he 

 speaks of aggressivity of this order as aggressivity in the narrower 

 sense, while he denotes the former as aggressivity in the wider sense of 

 the term. In their places one could substitute the terms active and 

 passive aggressivity, the latter indicating a passive resistance and the 

 former an actual offensive reaction. The general recognition of the 

 existence of a certain aggressivity on the part of the invading organ- 

 ism is most important. If in the past the attention of medical men 

 has been centred on the defensive mechanism of the invaded organism, 

 this interest has been essentially a selfish one. Active progress in 

 the future, however, will depend to a considerable degree upon our 

 knowledge of the defensive forces of the invader. Our present 

 knowledge is as yet quite small, but enough has been learned to 

 establish the importance of further research in this direction. 



Passive Aggressivity. CAPSULE FORMATION. Among the passive 



factors the most striking is the tendency to capsule formation, which 



occurs in some of the pathogenic bacteria, while they exist in the 



animal body, or when they are grown on media containing animal 



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