30 OFFENSIVE FORCES OF THE INVADING MICROORGANISM 



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 aggres- 

 sivity 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 evi- 

 dence 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 semipara- 

 sites, 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 would 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, and leads to a 

 fairly characteristic clinical picture of the corresponding infectious 

 diseases. 



Transition Forms. From the semiparasites the transition to the 

 necroparasites is represented by organisms like the bacillus of symp- 

 tomatic anthrax and the dysentery bacillus, both of which are toxin 

 producers, while their infectiousness and hence aggressivity are 

 slight. The streptococcus and pneumococcus, on the other hand, 

 stand midway between the semiparasites and true parasites, being 

 characterized by a considerable degree of infectiousness and a low 

 grade of toxicity. 



The remaining pathogenic organisms can be readily placed in this 

 system, the determining factors being their aggressivity (infectious- 

 ness) and their toxicity. The plague bacillus would thus find its 





