260 INFECTION AND IMMUNITY 



of the genus Anopheles, and that the malarial para- 

 site has been demonstrated not only in the blood of 

 those suffering from malarial infection, but also in 

 the stomach and salivary glands of the mosquito. If 

 the yellow-fever parasite resembled that of the ma- 

 larial fevers it would no doubt have been discovered 

 long ago. But as a matter of fact, this parasite, 

 which we now know is present in the blood of those 

 sick with the disease, has thus far eluded all re- 

 searches. Possibly it is ultra-microscopic. 



Individuals of every race and of all ages, who are 

 exposed to infection for the first time, during the 

 epidemic prevalence of the disease, are subject to be 

 attacked. But there is a wide difference in the de- 

 gree of this susceptibility among races, and among 

 individuals of the same race. 



It has been 'asserted that the negro race has a con- 

 genital immunity from yellow fever, but this is a 

 mistake. The susceptibility of the negro is, however, 

 much less than that of the white race, and among 

 those attacked the mortality, as a rule, is small. 



Immunity is acquired by suffering an attack of the 

 disease ; this acquired immunity is not, however, 

 absolute. 



Second attacks no doubt occasionally occur, al- 

 though this has been denied by some authors. 



The proper measures of prophylaxis are given in 



