VIRULENCE, INFECTIOUSNESS, OR AGGRESSIV1TY 31 



proper position close to the true parasites, while the staphylococcus, 

 meningococcus, and gonococcus would come somewhere between the 

 staphylococcus-pneumococcus group and the semiparasites proper, 

 and so on. It should be borne in mind, however, that the exact 

 position of an organism in this system may vary with different species 

 of animals, as least so far as its aggressivity is concerned. I have 

 pointed out already that the position given the cholera bacillus, for 

 example, is not exactly correct in the case of man, where it should 

 stand close to the necroparasites. The anthrax bacillus in the frog 

 and pigeon has ordinarily no aggressivity whatever, even though the 

 same strain may be most active in other mammals. The factors 

 which produce this difference in behavior are frequently unknown, 

 but sometimes, as in the last example, they are very simple; for in 

 this instance the apparently absolute resistance of the frog and 

 pigeon is referable to the fact that the anthrax bacillus ordinarily 

 does not grow readily at the temperature which is normal for the 

 animals in question. If, however, one gradually accustoms the 

 organism to those temperatures infection can be produced. 



Virulence, Inf ectiousness, or Aggressivity. From the foregoing survey 

 it is clear that the aggressivity of the pathogenic organisms 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 

 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; in some cases indeed 

 the organisms have already disappeared from the body at a time when 



