VIRULENCE, INFECTIOUSNESS, AND AGGRESSIV1TY 33 



differs very considerably, and the question naturally arises, To what 

 is this difference due? 



Clinically, we have long been in the habit of ascribing the varying / 

 severity observed in different cases of the various infectious diseases k ' 

 to differences in the virulence of the organism; in other words, the 

 severity of the clinical picture was regarded as an index of the severity 

 of the infection. This conception of the term is no longer tenable, 

 in view of our present knowledge of the relation or rather lack of 

 relation which exists between infection and infectious diseases; for, 

 as we have seen, the anthrax bacillus produces no evidence of disease 

 whatever until the end is almost at hand, although the blood may 

 be swarming with organisms long before. 



Using the term virulence in the old sense of the word, we would 

 accordingly be forced to look upon every anthrax infection as a non- 

 virulent infection, which would evidently be absurd. On the other 

 hand, we know that in tetanus serious symptoms of disease appear 

 relatively early, even though the bacillus multiplies to a slight extent 

 only, and the infection remains altogether local ;rin some cases, *y- 

 indeed, the organisms have already disappeared fromThe body at a / J 

 time when 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 vir- 

 ulence 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 recog- 

 nize 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 

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