190 INFECTION AND IMMUNITY 



Absolute immunity is exceedingly rare. The entire insusceptibility 

 of cold-blooded animals (frogs and turtles) under normal conditions to 

 inoculation with even the largest doses of many of the bacteria patho- 

 genic for warm-blooded animals, and the immunity of all the lower 

 animals against leprosy, are among the few instances of absolute immu- 

 nity known.' Apart from such exceptional cases, however, resistance, 

 immunity, and susceptibility must be regarded as purely relative terms. 



The power of resisting anj' specific infection may be the natural 

 heritage of a race or species, and is then spoken of as natural immunity. 

 It may, on the other hand, be acquired either accidentally or artificially 

 by a member of an ordinarily susceptible species, and is then called 

 acquired, immunity. 



Natural Immunity. — Species Immunity. — It is well known that many 

 of the infectious diseases which commonly affect man, do not, so far 

 as we know, occur spontaneously in animals. Thus, infection with B. 

 typhosus, the vibrio of cholera, or the meningococcus occurs in ani- 

 mals only after experimental inoculation. Gonorrheal and syphilitic 

 infection, furthermore, not only does not occur spontaneously, but is 

 produced experimentally in animals with the greatest difficulty — the 

 consequent diseases being incomparably milder than those occurring in 

 man. Other diseases, like leprosy, influenza, and the exanthemata,^ 

 have never been successfully transmitted to animals. ' 



Conversely, there are diseases among animals which do not spon- 

 taneously attack man. Thus, human beings enjoy immunity against 

 Rinderpest, and, to a lesser degree, against chicken cholera. 



Among animal species themselves great differences in susceptibility 

 and resistance toward the various infections exist. Often-quoted ex- 

 amples of this are the remarkable resistance to anthrax of rats and dogs, 

 and the immunity of the common fowl against tetanus. 



The factors which determine these differences of susceptibility and 

 resistance among the various species are not clearly understood. It 

 has been suggested that diet in some instances may influence these re- 

 lations, inasmuch as carnivorous animals are often highly resistant to 

 glanders, anthrax, and even tuberculous infections, to which herbiv- 

 orous animals are markedly susceptible.' It is likely, too, that the great 

 differences between animals of various species in their metabolism, 

 temperature, etc., may call for special cultural adaptation on the part 



Lubarsch, Zeit. f. klin. Mediz., xix. 

 With the possible exception of smallpox. 

 ' Hahn, in Kolle-und Wassermann, vol. iv. 



