INFECTION AND IMMUNITY 395 



nomenon, " if you take each of these attenuated cultures 

 as a starting-point for successive and uninterrupted 

 cultures, all this series of cultures will reproduce the 

 attenuated virulence of that which served as a starting 

 point; in this same way non-virulence will produce 

 non-virulence." 



"And, while hens who had never had chicken cholera 

 perished when exposed to the deadly virus, those who had 

 undergone attenuated inoculations and who afterward 

 received more than their share of the deadly virus, were 

 affected some with the disease in a benign form, a pass- 

 ing indisposition, sometimes, even, they remained per- 

 fectly well; they had acquired immunity. Was not 

 this fact worthy of being placed by the side of that 

 great fact of vaccine over which Pasteur had so often 

 pondered and meditated?" 



Practical application for the prevention of chicken 

 cholera was soon made of this observation, and it led to the 

 next great achievement in the way of inducing immunity. 



The bacillus of anthrax (splenic fever of cattle) had 

 been discovered by Davaine, and Pasteur's great ambi- 

 tion was to prepare some vaccine by which its ravages 

 might be stayed. The problem could not, however, 

 be so easily solved, for the spores of the bacillus prevented 

 its attenuation and preserved their original virulence 

 after having been kept dry for ten years. Clearly some 

 other means of attenuation must be devised. Eventu- 

 ally, after having tried many means of effecting the 

 attenuation necessary for the vaccine, he found that 

 when the microbes were cultivated at an elevated tem- 

 perature 42 to 43 C. they did not develop spores, 

 and that when cultures so modified were subsequently 

 cultivated at 30 C., they retained this peculiarity as 

 well as diminished virulence. 



The result of this observation was ability to produce 

 cultures of varying degrees of virulence, which like those 

 of the chicken-cholera microbes, bred true to their 

 acquired virulence. 



