34 OFFENSIVE FORCES OF THE INVADING MICROORGANISM 



of toxins, and one should accordingly speak of the toxicity of the 

 organism. Its virulence, i. e., its power to multiply in the body of the 

 infected animal, is always slight. Death is the outcome of its toxic 

 action, but not the expression of an especially high degree of virulence. 



The use of the latter term in infections with the necroparasites 

 would only be justifiable if one wished to give expression to the 

 idea that the severity of the clinical picture was due to the forma- 

 tion of an especially large quantity of toxin, which in turn would 

 indicate the presence of an especially large number of organisms. 

 This, however, scarcely enters into consideration, as we know that 

 the necroparasitic toxins are so extremely active that large numbers 

 of organisms are not at all needed to produce disastrous consequences, 

 after primary infection has once taken place. 



In infections with the true parasites, on the other hand, the term 

 virulence in its new sense is directly applicable; the more virulent 

 the organism the more readily will it multiply in the body of the 

 infected animal. In the semiparasites the term virulence may 

 occasionally still be applied in its original meaning, and the more 

 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. 



Differences 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 sur- 

 roundings by certain changes of a morphological or physiological 

 character, in consequence of which it becomes relatively or absolutely 

 unassailable 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 counter- 

 act or even to destroy the opposing forces of the host. 



