TRANSITION FORMS 31 



of the organism, without any evidence of disease. Such examples 

 form the exact counterpart to what we see in the case of the necro- 

 parasites, but here as there toxicity and infectiousness or aggressivity 

 bear an inverse relation to each other. We see, moreover, that 

 clinical manifestations of disease may be most pronounced on the 

 one hand, even though the infecting organisms have been unable 

 to maintain themselves in the body of the host (tetanus), while, 

 on the other, there may be the most extensive infection without 

 any evidence of a corresponding infectious disease. As Bail has 

 suggested, organisms belonging to this latter class may well be 

 looked upon as true parasites, whose aggressive mechanism must 

 evidently be of a different nature than that of the necroparasites 

 previously considered. 



Semiparasites. Between these two extremes stand the semi- 

 parasites, which are represented by the cholera vibrio and the typhoid 

 bacillus. Their infectiousness, and hence aggressivity, is already 

 quite well developed, although it is not comparable to what we see 

 in anthrax or chicken cholera, necessitating (in the animal experi- 

 ment) the introduction of a fairly large number of organisms and 

 often special methods of infection. In man the typhoid bacillus is 

 distinctly more aggressive than the cholera vibrio, which latter is 

 rarely found in the blood or tissues, although one w r ould imagine, 

 in view of the extensive epithelial desquamation and superficial 

 necroses, that opportunity for a general invasion would be readily 

 afforded. In addition to their aggressiveness the organisms of this 

 class are possessed of a well-marked toxicity, the effect of which 

 appears quite early in the course of the infection, but does not lead 

 to the production of specific symptoms, as we see them in the case 

 of the necroparasites. 



Transition Forms. From the semiparasites the transition to the 

 necroparasites is represented by certain anaerobic butyric acid 

 producing bacilli, such as the bacillus of malignant edema and the 

 bacillus of symptomatic anthrax, which are actively necrotizing 

 toxin producers, but possess a certain degree of aggressivity also, 

 as is evidenced by their wider distribution in the body of the infected 

 animal. Next in order follows the dysentery bacillus which behaves 

 as a typical semiparasite for the guinea-pig, while in the rabbit it is 

 relatively little infectious but exhibits a marked toxin production. 

 The streptococcus and pneumococcus, on the other hand, are closely 



