ATTENUATING THE VIRULENCE OF AN ORGANISM 321 



of the feeble virus. By this means the natural .resist- 

 ance of the animal is lowered, and the organism inocu- 

 lated is enabled to multiply and produce its pathogenic 

 effect, its virulence being subsequently exalted by 

 means of ''passages." 



Attenuating the Virulence of an Organism. Attenu- 

 ating or lowering the virulence of a pathogenic microbe 

 is usually attained with much less difficulty than the 

 exaltation of its virulence, and is generally effected 

 by varying the environment of the cultivations, as 

 for example: 



1. Cultivating in such media as are unsuitable by 

 reason of their (a) composition or (b) reaction. 



2. Cultivating in suitable media, but at an unsuitable 

 temperature. 



3 . Cultivating in suitable media, but in an unsuitable 

 atmosphere. 



4. Cultivation in suitable media, but under unfavor- 

 able conditions as to light, motion, etc. 



Attenuation of the virus can also be secured by 



5. Passage through naturally resistant animals. 



6. Exposure to desiccation. 



7. Exposure to gaseous disinfectants. 



8. By a combination of two or more of the above 

 methods. 



IMMUNISATION. 



The further study of the pathogenetic powers of any 

 particular bacterium involves the active immunisation 

 of one or more previously normal animals. This end may 

 be attained by various means; but it must be remem- 

 bered that immunisation is not carried out by any hard 

 and fast rule or by one method alone, but usually by a 

 combination of methods adapted to the exigencies of 

 each particular case. The ordinary methods include : 



A. Active Immunisation. 



I. By inoculation with dead bacteria (i. e., 



