Physiology of Pathogenic Microbes. 71 



It was at first supposed that the non-recurreuce of 

 infectious diseases was due to the fact that the mi- 

 crobes, at the time of the first attack, had abstracted 

 from the blood principles indispensable to their 

 growth, or, on the other hand, contaminated the 

 blood with principles which opposed their growth. 

 In short, two theories were entertained : that of ex- 

 haustion and that of impregnation. 



The first is to-day abandoned ; immunity has, in 

 fact, been overcome by employing large doses of 

 virus, which would be impossible if the organism 

 was really impoverished in substances indispensable 

 to the microbes. 



The doctrine of impregnation, on the other hand, is 

 strongly supported by the discovery of vaccinating 

 substances. A certain number of microbes secrete sub- 

 stances which mix with the fluids of the tissues, dif- 

 fuse through all the economy, and impair the vitality 

 even of the germs which have produced them ; these 

 preventive substances which appear not to be identical 

 with the toxic secretions of microbes oppose them- 

 selves to a recurrence of the disease. 



The existence of soluble vaccinating substances has 

 been unquestionably established for a certain number 

 of diseases : the blue pus disease, bacteridian charbon, 

 symptomatic charbon, cholera, Pasteur's septicaemia, 

 pneumo-enteritis of the pig, etc. 



Impregnation of the blood does not account for the 

 persistence of immunity. Substances prejudicial to the 

 growth of the germs must gradually become elimi- 

 nated from the economy, and if vaccination were due 

 solely to their presence, its effects would be extin- 

 guished in a comparatively short time. 



