466 IMMUNITY. 



ing it in a medium containing carbolic acid in the proportion 

 of i : 600. The virulence may also sometimes be attenuated 

 by injecting certain chemical substances along with the bacteria 

 into the body. Iodine terchloride was found by Behring to 

 modify in this way the virulence of the diphtheria bacillus. 



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. 



(f) By living Virulent Cultures in Non-lethal Doses. Immu- 

 nity may also be produced by employing virulent cultures in 

 small, that is non-lethal, doses. In subsequent inoculations the 

 doses may be increased in amount. For example, immunity 

 may thus be obtained in rabbits against the bacillus pyocyan- 

 eus. Such a method, however, has had a limited application in 

 the case of virulent organisms, as it has been found more con- 

 venient to commence the process by attenuated cultures. 



Exaltation of the Virulence. The converse process to attenua- 

 tion, i.e. the exaltation of the virulence, is obtained chiefly by the 

 method of cultivating the organism from animal to animal the 

 method of passage discovered by Pasteur (first, we believe, in 

 the case of an organism obtained from the saliva in hydro- 

 phobia, though having no causal relationship to that disease). 

 This is most conveniently done by intraperitoneal injections, as 

 there is less risk of contamination. The organisms in the peri- 

 toneal fluid may be used for the subsequent injection, or a cul- 

 ture may be made between each inoculation. The virulence of 

 a great number of organisms can be increased in this way, the 

 animals most frequently used being rabbits and guinea-pigs. 

 This method can be applied to the organisms of typhoid, 

 cholera, pneumonia, to streptococci, and staphylococci, and in 

 fact to those organisms generally which invade the tissues. 



The virulence of an organism, especially when in a relatively 

 attenuated condition, can also be raised by injecting along with 

 it a quantity of a culture of another organism either in the living 

 or dead condition. A few examples may be mentioned. An 

 attenuated diphtheria culture may have its virulence raised by 

 being injected into an animal along with the streptococcus 

 pyogenes ; an attenuated culture of the bacillus of malignant 



