32 OFFENSIVE FORCES OF THE 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 what- 

 ever, even though the same strain may be most active in other mam- 

 mals. 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 temperatures 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 macro organism 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 



