ADJUSTMENT TO INFECTIOUS DISEASE 409 



Every species of higher animal has its pecuHar invaders, 

 which rarely or never cause spontaneous infections in 

 other species. Many diseases of man cannot be inoculated to 

 any animals except the higher apes; and domestic animals 

 suffer from fatal infections which often have no power what- 

 ever to invade man. Moreover, in certain instances, rabies 

 for example, where an invader can afflict many different 

 species, continued passage through one type of host (rabbits) 

 will increase the virulence for this one, considerably 

 diminishing its invasive power for man. Organisms Hke the 

 gonococcus, which spontaneously infect man only, exhibit 

 a distinctly nutritive adaptation by refusing to grow on 

 anything but human protein in the first generations of 

 artificial cultivation, rapidly losing this fastidiousness after 

 a short time of test-tube existence. 



While these and many other examples that might be 

 given illustrate the dehcacy of adaptation to the invasive 

 from the saprophytic condition, still more remarkable is 

 the speed and ease with which, in the laboratory, we can 

 increase or decrease the virulence or invasive powers of 

 certain bacteria by the simplest expedients. With the 

 pneumococcus, for instance, we can readily, by successive 

 mouse passage, enhance virulence until one-miflionth of a 

 broth culture wifl kill; and by properly manipulating this 

 same culture, we can obtain individual so-called "rough" 

 colonies, which are typical in most of their biological proper- 

 ties, but win no longer kiO mice except in large doses. It is 

 becoming increasingly apparent that the study of bacterial 

 mutation is promising to develop facts of profound biological 

 importance since, together with the loss of virulence, changes 

 may occur in cefl chemistry and immunological properties. 



Since every infection is of course a process in which the 

 two chief variables concerned are the pathogenic micro- 

 organism and the host, it has been necessary to discuss 

 briefly the factors influencing the invader. We are chiefly 

 concerned in this chapter, however, with the conditions of the 

 host by which invasion is either permitted or prevented. 

 The higher animals all possess in their normal state a 

 so-called "natural" resistance against many bacteria. 

 This natural resistance can to some extent be analyzed into 



