222 INFECTION AND IMMUNITY 



varicella, or chicken-pox, should be borne in mind. 

 This mistake has been responsible for the spread of 

 smallpox in numerous instances and is to be guarded 

 against by the isolation of the sick and the measures 

 of disinfection heretofore recommended for prevent- 

 ing the extension of that disease. The period of 

 incubation in chicken-pox is fourteen days. The 

 germ of this disease has not been discovered, but it 

 has been demonstrated that it is present in the 

 " lymph " contained in the vesicles which are charac- 

 teristic of the disease. 



