92 PATHOGENIC BACTERIA. 



serpents and insects. For this reason considerable ref- 

 erence must be made in the treatment of the subject to 

 the phenomena associated with these poisons. 



Immunity is always a relative condition. Carl Fraen- 

 kel expressed this admirably when he said, u a white 

 rat is immune to anthrax in amounts sufficiently large to 

 kill a rabbit, but it is perhaps not immune to a quantity 

 sufficiently large to kill an elephant." The fowl can 

 overcome as much toxin as the tetanus bacilli can pro- 

 duce before their destruction in its body, but cannot 

 overcome the effects of an unlimited quantity of the 

 poison. The hedge-hog is immune to serpent's venom 

 in the doses usually injected by the snakes, yet can be 

 killed by the venom in larger doses. Many animals are 

 immune to as many bacteria as reach them in the usual 

 modes of infection, but will succumb to excessively large 

 doses of the same bacteria artificially introduced. 



The standard of immunity may be expressed as the re- 

 sistance manifested by the normal, healthy animal to the 

 unmodified germs of disease. 



In attempting to establish that any animal is immune 

 it is of the utmost importance to bear in mind that the 

 virulence of bacteria is subject to the greatest variation 

 under purely natural conditions. A few bacteria, as the 

 anthrax bacillus, maintain a definite standard for long 

 periods without observable attenuation, and may be 

 manipulated artifically in the usual ways without exalta- 

 tion of virulence. A greater number, of which the strep- 

 tococcus and pneumococcus will serve as illustrations, 

 are so variable that it is unusual for two organisms from 

 different sources to have the same degree of virulence, or 

 for any one organism to have the same degree of virulence 

 for any considerable length of time. The meaning in- 

 tended by the expression " unmodified germs of disease " 

 is the natural virulence, without modification or manip- 

 ulation in the laboratory. 



