32 OFFENSIVE FORCES OF THE INVADING MICROORGANISM 



the patient is dying from the effect of their toxins. Such an infection 

 one would be apt to look upon as especially virulent. Evidently 

 the severity of the clinical pictures is no index of the virulence of the 

 organism. The confusion is altogether due to the fact, that in the 

 past the toxic power of an organism and its infectious power have 

 been looked upon as synonymous, while we now recognize that the 

 two are separate factors. 



The toxicity of an organism is in a measure an accidental property 

 which is of interest from the fact that it is responsible for certain 

 symptoms of the infection, but it is not by any means essential to 

 infection. This is shown especially well in the case of the tetanus 

 bacillus, whose toxins by themselves, after separation from the 

 organism, are capable of producing the identical clinical picture 

 which follows actual infection. 



The term virulence, in its modern meaning, has reference essen- 

 tially to the ability of an organism to multiply in the body of the 

 infected animal, and is hence virtually synonymous with infectiousness 

 or aggressivity. It is hence erroneous to speak of the virulence of a 

 tetanus bacillus or a diphtheria bacillus to indicate the severity 

 of a given case; the clinical picture is essentially due to the action 

 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 indeed 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 toxins, 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 semi parasites the term virulence may 

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



