ACTIVE IMMUNITY 553 



attenuated cultures could be used for protective inoculation. He 

 considered the loss of virulence to be due to the action of the 

 oxygen of the air, as he found that in tubes sealed in the absence 

 of oxygen the virulence was not lost. Haffkine attenuated cul- 

 tures of the cholera spirillum by growing them in a current of 

 air (p. 472). 



(2) The virulence of an organism for a particular animal may 

 be lessened by passing the organism through the body of another 

 animal. Duguid and Burdon Sanderson found that the virulence 

 of the anthrax bacillus for bovine animals was lessened by its 

 being passed through guinea-*pigs, the disease produced in the 

 ox by inoculation from the guinea-pig being a non-fatal one. 

 This discovery was confirmed by Greenfield, who showed that 

 the bacilli cultivated from guinea-pigs preserved their property 

 in cultures, and could therefore be used for protective inoculation 

 of cattle. A similar principle was applied in the case of swine 

 plague by Pasteur, who found that if the organism producing 

 this disease was inoculated from rabbit to rabbit, its virulence 

 was increased for rabbits but was diminished for pigs. The 

 method of vaccination against smallpox depends upon the same 

 principle. There is also evidence to show that the virulence of 

 the tubercle bacillus becomes modified according to its host, 

 being often diminished for other animals. 



(3) Many organisms become diminished in virulence when 

 grown at an abnormally high temperature. The method of 

 Pasteur, already described (p. 356), for producing immunity 

 in sheep against anthrax bacilli, depends upon this fact. A 

 virulent organism may also be attenuated by being exposed to 

 an elevated temperature which is insufficient to kill it, as was 

 found by Toussaint in the case of anthrax. 



(4) Still another method may be mentioned, namely, the 

 attenuation of the virulence by growing the organism in the 

 presence of weak antiseptics. Chamberland and Roux, for 

 example, succeeded in attenuating the anthrax bacillus by 

 growing it in a medium containing carbolic acid in the propor- 

 tion of 1 : 600. 



These examples will serve to show the principles underlying 

 attenuation of the virulence of an organism. There are, how- 

 ever, still other methods, most of which consist in growing the 

 organism in conditions somewhat unfavourable to its growth, e.g., 

 under compressed air, etc. 



(b) Immunity by living Virulent Cultures in Non-lethal 

 Doses. Immunity may also be produced by employing virulent 

 cultures in small, that is non-lethal, doses. In subsequent 



