64 INFECTION AND IMMUNITY 



it an exclusively vegetable diet. Again, natural im- 

 munity may in some cases be overcome by the de- 

 vitalising agencies mentioned in the chapter on sus- 

 ceptibility to infection (starvation, great fatigue, etc.). 

 Infection also depends upon the comparative virul- 

 ence of the infecting agent, or germ, and to some 

 extent upon the number of germs introduced. 



Immunity, therefore, whether natural or acquired, 

 often has only a relative value, and may be overcome 

 as a result of circumstances favourable to infection. 

 Thus it has been found that germs having very little 

 pathogenic virulence, and harmless under ordinary 

 conditions, may kill guinea-pigs when injected into 

 the muscles of the thigh after they have been bruised 

 by mechanical violence. Pasteur found that fowls, 

 which have a natural immunity against anthrax, be- 

 come infected and die if they are subjected to artificial 

 refrigeration after inoculation. Pigeons have a natural 

 immunity against anthrax, but if they are enfeebled 

 by lack of food they succumb to inoculations with 

 the anthrax bacillus. 



The pathogenic power of known disease germs 

 also varies greatly as a result of conditions relating 

 to their development. In general it may be said that 

 cultivation in the bodies of susceptible animals in- 

 creases the virulence of disease germs. Attenuation 

 of virulence may be effected by several methods, all 



