554 IMMUNITY 



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 only a 

 limited application, as it has been found more convenient to 

 commence the process with dead or attenuated cultures, and then 

 to continue with virulent 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 

 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 staphylococci, and in fact to 

 those organisms generally which invade 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 

 oedema by being injected with the bacillus prodigiosus ; an 

 attenuated streptococcus by being injected with the bacillus coli, 

 etc. A culture of the typhoid bacillus may be increased in 

 virulence, as already stated, by being injected along with a dead 

 culture of the bacillus coli. In such cases the accompanying 

 injection enables the attenuated organism to gain a foothold in 

 the tissues, and it may be stated as a general rule that the 

 virulence of an organism for a particular animal is raised by its 

 growing in the tissues of that animal. 



Combination of Methods. The above methods may be com- 

 bined in various ways. By repeated injections of cultures at 

 first in the dead condition, then living and attenuated and 

 afterwards more virulent, and by increasing the doses, a high 

 degree of immunity may be obtained. 



2. Immunity by Dead Cultures of Bacteria. In some cases 

 a high degree of immunity against infection by a given microbe 



