194 INFECTION AND IMMUNITY 



accidentally discovered that the virulence of the bacilli of this disease 

 was greatly reduced by prolonged cultivation upon artificial media. 

 This was especially noticeable in broth. cultures which had been stored 

 for long periods without transplantation. By repeated injections of such 

 cultures into fowl, he succeeded in rendering the animals immune against 

 subsequent inoculations with lethal doses of fully virulent strains. 



During the same year, 1880, in which Pasteur published his observa- 

 tions on chicken cholera, Toussaint ' succeeded in immunizing sheep 

 against anthrax by inoculating them with blood from infected animals, 

 defibrinated and heated to 55° C. for ten minutes. Toussaint wrongly 

 believed, however, that the blood which had been used in his immuniza- 

 tions was free from Hving bacteria. In repeating this work Pasteur 

 showed that the protection in Toussaint's cases was conferred by living 

 bacteria, the virulence of which had been reduced by their subjection to 

 heat. 



In following out the suggestion^ offered by these experiments, 

 Pasteur^ discovered that he could reduce the virulence of anthrax 

 bacilli much more rehably than by Toussaint's method, by cultivating 

 the organisms at increased temperatures (42° to 43° C.) . By this process 

 of attenuation he was able to produce " vaccines " of roughly measurable 

 strength, with which he succeeded in immunizing sheep and cattle. 

 A successful demonstration of his discovery was made by him at Pouilly- 

 le-Fort, soon after, upon a large number of animals and before a commis- 

 sion of professional men. 



It is a fact well known to bacteriologists that certain of the pathogenic 

 microorganisms, when passed through several individuals of the same 

 animal species, become gradually more virulent for this species. In his 

 studies on the bacillus of hog cholera, Pasteur observed that when this 

 microorganism was passed through the bodies of several rabbits it gained 

 in virulence for rabbits, but became less potent against hogs. He suc- 

 ceeded, subsequently, in protecting hogs against fully virulent cultures 

 by treating them with strains which had been attenuated by their 

 passage through rabbits. 



A further principle of attenuation for purposes of immunization was, 

 at about this time, contributed by Chamberland and Roux,* who re- 



' Toussaint, Compt. rend, de I'acad. des sci., 1880, t. xci. 



2 Pasteur, Chamberland et Roux, Compt. reiMl. de I'acad. des sci., 1881, t. xeii. 



= Pasteur, Compt. rend, de I'acad. des sci., 1882, t. xcv. 



* Chamberland et Roux, Compt. rend, de I'acad. des sci., 1882, t. xcvi. 



