DEFENSIVE FACTORS OF THE ANIMAL ORGANISM 241 



naturally immune hens susceptible to anthrax by cooling them to a 

 subnormal temperature, and Canalis and Morpurgo did the same with 

 doves by subjecting them to starvation. 



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 

 pathogenic for warm-blooded animals, and the immunity of all the 

 lower animals against leprosy, are among the few instances of absolute 

 immunity known. 2 Apart from such exceptional cases, however, re- 

 sistance, immunity, and susceptibility must be regarded as purely 

 relative terms. 



The power of resisting any specific infection may be the natural 

 heritage of a race or species, and is then spoken of as natural im- 

 munity. It may, on the other hand, he 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 animals only after experimental inoculation. Gonorrheal and 

 syphilitic infection, furthermore, not only does not occur spontane- 

 ously, 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, 3 have never been sucessfully 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 

 examples 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 

 relations, inasmuch as carnivorous animals are often highly resistant 



2 Lubarsch, Zeit. f. klin. Medi/., xix. 

 'With the possible exception of smallpox. 



