430 IMMUNITY. 



growing 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 under- 

 lying attenuation of the virulence of an organism. There 

 are, however, still other methods, most of which consist in 

 growing the organism in conditions somewhat unfavourable 

 to its growth, e.g., under compressed air, etc. 



(1)) 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 

 inoculations the doses may be increased in amount. For 

 example, immunity may thus be obtained in rabbits against 

 the bacillus pyocyaneus. Such a method, however, has 

 had a less wide application, as it has been found more 

 convenient to commence the process by attenuated cultures. 



Exaltation of the Virulence. The converse process to 

 attenuation, 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 hydrophobia, though having no 

 causal relationship to that disease). This is most con- 

 veniently done by intraperitoneal injections, as there is less 

 risk of contamination. The organisms in the peritoneal 

 fluid may be used for the subsequent injection, or a culture 

 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 staphylo- 

 cocci, and in fact to those organisms generally which invade 

 the tissues. 



The virulence of an organism, especially when in a more 

 or less attenuated condition, can also be raised by injecting 



