32 OFFENSIVE FORCES OF INVADING MICROORGANISM 



related to the 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 

 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, at 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 at the temperature which is normal for the animals in question. 

 If, however, one gradually accustoms the organism to those tem- 

 peratures infection can be produced. 



Tissue Parasites. It will be noted that no mention has been made 

 of the position of either the tubercle bacillus or of actinomyces in 

 the above schema. As a matter of fact, these organisms occupy 

 a position of their own, being essentially tissue parasites, while the 

 others may be looked upon as humoral parasites. Their behavior 

 in the macroorganism is in every respect different from that of the 

 remainder. While the latter only affect a certain group of cells 

 (i. e., the leukocytes) in a direct manner, the tissue parasites bring 

 about an altered response of the body at large, i. e., an allergia 

 which is in a certain sense characteristic of this group. This peculiar 

 behavior is also shown by one of the animal parasites, viz., the 

 Treponema pallidum, while the trypanosomes resemble the humoral 

 bacterial parasites (see also section on Allergia). 



Virulence, Infectiousness, and Aggressivity. From the foregoing 

 survey it is clear that the aggressivity of the pathogenic organisms 



