ATTENUATION OF VIRUS. 203 



the cultivation, at short intervals, of the different 

 grades of " attenuated virus." 



Subsequent experiments with the virus of an- 

 thrax (charbon) gave similar results ; and, under the 

 direction of Pasteur, extensive protective inocula- 

 tions have been practised in France with attenu- 

 ated virus prepared by this method. 



The time of exposure to oxygen is less for the 

 anthrax bacillus than is required in the case of the 

 micrococcus of fowl-cholera ; and it is necessary 

 to cultivate the bacillus in such a way as to pre- 

 vent the development of spores, as these retain 

 their virulence unchanged for many years. This 

 is accomplished by cultivating the bacillus at a 

 temperature of 42 to 43 C., at which point no 

 spores are developed, the organism multiplying by 

 fission only. Contact with the atmosphere for a 

 month destroys entirely the vitality of the bacillus 

 in such a culture, and in eight days it loses its 

 deadly properties, the temperature being main- 

 tained at the point mentioned. During this time 

 the virus passes through successive degrees of at- 

 tenuation. It is possible to restore the mitigated 

 virus to its full activity by inoculating a guinea- 

 pig one day old, which is killed by the operation, 

 and using the blood of this animal to inoculate a 

 second ; and so on. After repeating this operation 

 several times, the poison is said by Pasteur to re- 

 gain its full vigor, and to be fatal to a sheep. In 

 the same way the attenuated virus of fowl-cholera 

 may be restored to full vigor by inoculating a 



