DAWN OF A NEW ERA IN MEDICINE 197 



that when inoculated into pigeons it becomes in- 

 creased in virulence as it becomes transferred from 

 pigeon to pigeon, killing the birds more and more 

 quickly the longer the inoculations are continued. 

 Introduced from pigeons to the pig the germs be- 

 came more frequently fatal to these animals, thus 

 proving that they had become more virulent while 

 in the bodies of the birds. 



On the other hand, it was found that in succes- 

 sive inoculations into rabbits the severity of the 

 disease becomes reduced. When the germs that 

 have been passed through a series of rabbits are 

 introduced into the pig they produce only a mild 

 attack of the disease, and after recovery the ani- 

 mals are immune to future attacks. The treatment 

 founded on these experiments has proven success- 

 ful in checking swine plague, which is a very 

 destructive malady. Unfortunately swine plague 

 was at first confused with hog cholera, a deadly 

 disease, now known to be caused by a filterable 

 virus and which has been greatly reduced by the 

 administration of a protective serum. The fact 

 demonstrated by these and other experiments that 

 virulence is not a fixed property of micro-organ- 

 isms, but something which may be increased or 

 decreased, and to a certain extent manipulated at 



