72 EVOLUTION OF THE INFECTIOUS DISEASES 



quantity of normal, antibodies or those which preexist in 

 excess; in other words, by the degree of immunity, natural 

 or acquired. 



3. By taking into consideration: 



(a) The frequency of these different bacteria in nature 

 and consequently the frequency of infections and of possible 

 spontaneous vaccinations : 



(6) The gastro-intestinal digestibility of bacterial bodies 

 and of the products of their cleavage. 



(c) The permeability of the intestinal mucosa for these 

 products. 



(d) Finally the digestibility of these products which have 

 passed as antigens into the interior of the organism ; we may 

 classify these organisms in the following way: 



1. The bacilli of Eberth are the least frequent, giving the 

 poorest growths in all the known culture media. The prod- 

 ucts of their proteolysis are most difficult to digest in the 

 digestive apparatus of man. The products of bacteriolysis 

 always begin by penetrating in small quantity (immunizing 

 quantity) into the blood where they stay during a relatively 

 long incubation period. The compounds of the antigen 

 which have penetrated into the blood with their antibodies 

 are digestible with difficulty in the interior of the organism; 

 on account of which lesions are severe and the period of 

 disease is relatively long. 



2. Paratyphoid bacilli are more frequent, giving more 

 rapidly most potent cultures, are perhaps more proteolytic 

 and more difficult of digestion for certain animals (hog 

 cholera, typhoid of mice, psittacosis) and spontaneous 

 vaccination is more frequent. The products of bacteriolysis 

 may penetrate from the intestines into the blood in massive 

 doses and cause rapid crises of considerable violence but of 

 short duration, since the subject contains a larger quantity 

 of antibody in his blood and in his tissues, or, in other words, 

 is more strongly immunized. The compounds of antigen with 

 antibody are more easily digested and in consequence less 

 pathogenic than those of typhoid. 



3. Colon bacilli are the most widely distributed of this 

 group of organisms and the easiest to cultivate. The prod- 



