EXANTHEMATA 123 



develop lesions very similar to cow-pox. In monkeys 

 inoculation with cow-pox virus protects against small- 

 pox, and vice versa, and the same protective action 

 holds in man. This has been proved for more than a 

 century i. e., since Jenner, in 1796, introduced vaccina- 

 tion. In every country where vaccination has been made 

 compulsory there has been a sudden and constant lower- 

 ing of the number of small-pox cases, and those which do 

 occur are of a milder nature than formerly. 



