IMMUNITY. 303 



Another method of attenuation based in this 

 case upon utilization of the living animal body 

 was also first used by Pasteur. If the bacteria 

 of swine-erysipelas are transferred from swine, 

 in which they possess a definite degree of vir- 

 ulence, to pigeons, and thence again to fresh 

 pigeons, they acquire in the course of this 

 transfer a heightening of their virulence. Con- 

 versely, a diminution of virulence is brought 

 about by transfer from rabbit to rabbit. By 

 use of the bacteria which have become weak- 

 ened in their passage through the body of the 

 rabbit it is possible to protect against that 

 degree of virulence shown in spontaneous 

 swine-erysipelas ; or it is even possible to go a 

 step farther and heighten the artificial im- 

 munity so that it protects against the germ 

 whose virulence has been increased artificially 

 by passage through the body of the pigeon. 



Pasteur discovered a similar thing in rabies. 

 The virus, from which a definite microbe has 

 not yet been isolated, becomes weaker (the 

 degree of virulence shown in rabid dogs being 

 designated as " street-virus ") by passage 

 through the ape, stronger by passage through 

 the rabbit. Pasteur made use of the spinal 

 cord of rabbits in which virulence had been 

 exalted to the highest point, and called this 

 strongest obtainable virus " virus fixe." In 



