ACTIVE IMMUNITY. 463 



ment of cultures of the living organisms, the virulence of 

 which was so diminished that on inoculation they did not 

 produce a fatal disease, but yet had effects sufficient for 

 protection. The principle is therefore the same as that 

 of vaccination, and the attenuated cultures are often 

 spoken of as vaccines. The virulence of an organism may 

 be diminished in various ways, of which the following 

 examples may be given. 



(1) In the first place, practically every organism when 

 cultivated for some time outside the body, loses its virulence, 

 and in the case of some this is very marked indeed, e.g., the 

 pneumococcus. Pasteur found in the case of chicken 

 cholera, that when cultures were kept for a long time in 

 ordinary conditions, they gradually lost their virulence, and 

 that when sub-cultures were made, the diminished virulence 

 persisted. Such attenuated cultures could be used for pro- 

 tective 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 cultures of 

 the cholera spirillum by growing them in a current of air. 



(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 being passed through guinea-pigs, the dis- 

 ease produced in the ox by inoculation from the guinea-pig 

 being a non-fatal one. This discovery was confirmed by 

 Greenfield, who found 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. 

 Organisms which had been passed through a series of 

 rabbits produced in the pig illness, but not death, and 

 protection for at least a year resulted. The method 



