138 THEORIES CONCERNING IMMUNITY 



these diseases. It was hoped that in the course of this 

 research it would become possible to explain at the same time 

 the mechanism of all these reactions namely, why a bac- 

 terium is pathogenic or may become so, how and why an 

 infected organism may spontaneously recover or succumb 

 to the disease, whether all infectious diseases have the same 

 evolution and also why the reactions caused by bacteria 

 or by their toxins differed from the reactions caused by most 

 toxic salts, inorganic or organic. 



In reality these investigations have met with difficulties 

 as numerous as they were unforeseen. Even today, after 

 half a century of relentless labor, there are very contagious 

 diseases, such as measles, scarlatina, typhus, hydrophobia, of 

 which the germs are unknown; there are others such as leprosy 

 malaria, the trypanosomiases, of which the germs are known 

 but which cannot yet be cultivated on artificial media; there 

 are others for which we have not succeeded in preparing either 

 preventive vaccine or curative serum; lastly, there are others 

 (psoriasis, eczema, cancer, etc.) which are not contagious 

 and of which the germs are unknown, but which have many 

 characteristics in common with diseases of infectious origin. 



It was therefore necessary to recognize that all these 

 problems could not be solved by frontal attacks; that if, in 

 certain cases, it was possible to arrive at practical solutions 

 by avoiding obstacles, and that if it was possible to discover 

 preventive vaccines or curative treatments without knowing 

 the causes of the disease Jenner for small-pox and Pasteur 

 for hydrophobia in many other cases it was necessary, in 

 order to arrive at the same results, to make a long detour 

 in the attempt to better understand the functions of the 

 organism, and the nature and mechanism of its normal and 

 pathologic reactions. 



And thus it is that nearly all the exact and biological 

 sciences, chemistry and physics, general biology together 

 with medicine and bacteriology, have been drawn upon to 

 attack the immunity problem from several angles at the 

 same time. What often happens in the exploration of 

 unknown lands has occurred here: In following different 

 trails, the explorers lose sight of each other at times, their 



