128 IIISTOKICAL SURVEY OF YELLOW FEVEK 



chapter, yellow fever has become rarer in the yellow 

 fever zone of the world. This means that the immunising 

 process which used to occur in the old days no longer 

 takes place now, and e\ery year adds to the number 

 of native non-innnunes ; they no longer get the oppor- 

 tunity of acquiring the disease. In other words, the 

 black has now become as susceptible as the latest white 

 arrival to yellow fever. This is, at any rate, my 

 interpretation of why the natixe barbadian is suffering 

 from yellow fever equally with the new arri\als. 



Pei'iodicitfi. — This has been referred to by many 

 writers, and it is often stated that in the case of 

 Barbados the island is \isitcd at intervals of thirteen 

 years. Here again the coincidence is probably de- 

 pendent upon the creation of a larger susceptible 

 population during the interval of freedom from disease, 

 the immunes having diminished in the meantime 

 through natural causes, as by death. 



I 



