94 Manual of Veterinary Microbiology. 



durable, and persists in later generations of these 

 microbes. 



The methods which furnish an hereditary attenu- 

 ation permit of obtaining more easily large quantities 

 of attenuated virus, and are more especially utilized 

 in the preparation of vaccines. 



III. Preventive inoculations. Vaccinations. 



The organism may be made refractory to a bac- 

 terial disease by diiferent means : by inoculating the 

 natural virus, the attenuated virus, a chemical vaccine, 

 or, finally, a virus difierent from that against which 

 it is desired to fortify the organism. 



1. Preventive inoculation of iiaiural virus. — It has 

 been observed that a certain number of contagious 

 diseases leave behind them in the subject, after re- 

 covery, a solid immunity against these same diseases. 

 On the other hand, certain diseases purposely com- 

 municated show themselves much less dangerous than 

 when they prevail naturally. The recognition of 

 these facts has given origin to variolization, clavel- 

 ization,* preventive inoculation against pleuro-pneu- 

 monia, etc. The first, for more than a century, has 

 been replaced by vaccination ; the second is still 

 recommended in our times, in default of a means of 

 prevention more practical, if not more efiicacious. 



In this case the only object is to produce a disease 

 of the same nature as the spontaneous disease, but 

 mild in character, not threatening the life of the in- 

 dividual, yet endowing it with immunity. This end 

 is obtained by diminishing the number of the germs 



* [Artificial infection of flocks with ovine variola. — D.] 



