A Round-up of Undesirables 133 



known since it has not been artificially culti- 

 vated and so cannot be studied free from 

 complicating conditions. 



The infective organism is naturally limited 

 to the human race, though monkeys can be 

 experimentally infected. 



Except for the few instances in which by 

 accident the infective agent has been conveyed, 

 it is seldom known to be a source of danger to 

 those of honorable life, except through inti- 

 mate association with its victims. 



The Plague 



The plague, or "black death" as they called 

 it when it ravaged Europe in the middle ages, 

 has a very ancient and sinister history. You 

 may read about it in the pages of Bocaccio 

 or Defoe. It had largely disappeared from 

 Europe for a century and a half. But in the 

 latter part of 1900 it started afresh in China 

 and India, and gradually worked its way into 

 the Americas. It has never fairly gained a 

 foothold in the United States, thanks to 

 effective quarantine, except in San Francisco, 



