508 Chapter XVI 



protected against small-pox. Being a man of great understanding and 

 culture, he set himself to verify this opinion experimentally. Having 

 [531] demonstrated by a great number of experiments that the inoculation of 

 variolous virus into persons vaccinated by cow-pox had no ill result, 

 he became the great propagandist of the new method. He worked 

 at this subject for 20 years but only decided to publish his results (in 

 1798) after he had completely satisfied himself of the great utility 

 of vaccination with the virus of cow-pox. At first Jenner's discovery 

 met with great opposition, but his method was soon verified in 

 France and several other countries and it was not long before it was 

 generally practised. 



When Pasteur set himself to study the infective diseases in 

 their relation to micro-organisms the idea of profiting by the 

 discovery of these pathogenic organisms and of drawing from them 

 a weapon against infections soon arose in his mind. He studied 

 Jenner's work in order to extract from it any indications capable 

 of putting him into the right path. He induced his collaborators 

 to carry out several series of experiments with the object of immu- 

 nising the animal organism against infective micro-organisms. During 

 this laborious and original work chance 1 helped in the accom- 

 plishment of his task. When, at the conclusion of the holidays in 

 the autumn of 1879, Pasteur and his collaborators Chamberland and 

 Roux wished to resume their experiments on fowl cholera, they 

 found to their great surprise that the micro-organisms of this disease, 

 usually so fatal, had become innocuous. Fowls, that received doses 

 of cultures much more than sufficient to cause death, did not 

 experience any ill effect. Prepared by his previous knowledge and 

 by the continual direction of his thoughts to the prevention 

 of contagious diseases, Pasteur divined at once the great 

 bearing of this check in his inoculations with old cultures, and 

 immediately began to make precise experiments as to the 

 vaccinating power of these micro-organisms which had become 

 innocuous. These researches led him to the discovery of two great 

 principles: that of the attenuation of viruses, and that of the 

 vaccinating property of attenuated micro-organisms. Various me- 

 moirs by Pasteur 2 established these laws in a very exact manner ; 

 moreover he gave all the information necessary to allow of the 



1 See Vallery-Radot, "La Vie de Pasteur," Paris, 1900, p. 427. 



2 Compt. rend. Acad. d. sc., Paris, 1880, t. xc, pp. 939, 952, 1030; t. xcr, pp. 571, 

 673. 



