240 LOUIS PASTEUR 



of enquiries to which the experiments pointed, as 

 he was then chiefly interested in establishing the 

 important principle of attenuation. This was left 

 for others, and the investigations thus started and 

 continued in the Pasteur Institute and elsewhere 

 have led, step by step, to the most striking results 

 in serum therapy and allied procedures. 



In 1894, Yersin, who had gone from the Pasteur 

 Institute to China, discovered the germ of the 

 plague, and found that it could be cultured in arti- 

 ficial media, and inoculated into rats, mice, and 

 guinea pigs. The subsequent discovery of the role 

 played by rats in harboring the germs of the plague, 

 and its transmission from animal to man and from 

 man to man by fleas, has made it possible to stamp 

 out this disease in several places in which it had 

 gained a foothold. The epidemics that formerly 

 swept over nations carrying away thousands in 

 their course may now be controlled. Where plague 

 occurs war is made on the rats and fleas, the pa- 

 tients are strictly quarantined and the epidemic 

 dies. 



Pasteur was profoundly gratified by the discov- 

 eries of his co-workers. They were bringing nearer 

 to realization his dream of the conquest of disease 

 though the methods which he had discovered and 



