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The immunity in the three first classes is, as a rule, absolute, so long 

 as the persons continue to reside in yellow fever countries, but it may be 

 partially or totally lost through an absence from such countries, or even 

 by a mere withdrawal from chances of infection, during several consecutive 

 epidemic seasons (probably not less than four or five years). In the fourth 

 class, on the other hand, the immunity is often only partial, "acclimation 

 fevers", or mild albuminuric forms, being still occasionally observed on 

 severe exposure; but when, as exceptions, severe attacks occur among 

 them, the chances of recovery are not much greater than they are in 

 ordinary cases. Even without the condition of prolonged residence, ex- 

 perience also teaches that after a single attack of "acclimation fever" there 

 are great probabilities that beyond a possible repetition of the same 

 form, or, more rarely, a mild albuminuric attack little danger will be 

 incurred by continuing to reside in the place. 



As regards the immunity observed among the whites who have been 

 born and bred in Havana, it cannot be considered as hereditary, for other 

 children of the same parents, born in countries where yellow fever is 

 unknown, do not partake of the same exemption. Among native children 

 casas of "acclimation fever" probably often occur, though not diagnosed 

 as such, and in the outskirts of the city, or, more rarely, in the city itself, 

 children are not unfrequently attacked with a form of fever ("fiebre de 

 borras") in which "black vomit" and albuminuria are common symptoms. 

 It seems probable, therefore, that in many instances the immunity observ- 

 ed in the white natives must be attributed to mild attacks experienced ¡a 

 early life. There is reason to believe, however, that it is sometimes acquired 

 through an attack suffered in utero, in consequence of infections trans- 

 mitted by immune mothers; for in two instances within my knowledge 

 immune mothers have given birth to children who had fever at the time they 

 were brought into the world, and only recovered after a few days. In one 

 of these cases the mother had been attending a severe case of yellow fever 

 during the last month of her pregnacy in the very room in which her 

 child was born; and in the other, the mother lived in a district where 

 several cases of yellow fever had recently occurred. 



In the uncertainty that still surrounds questions of immunity, all 

 conjectures must be hazardous. I beg, however, to submit the following 

 suggestions, even if they serve no other purpose beyond linking together 

 the peculiarities which are actually observed in reference to yellow fever. 



The trasmission of the disease through an immune mother to I lie 

 foetus in utero shows that the immunity does not exclude the introduction 

 and circulation of the specific germs through the blood vessels or lympha- 

 tics, but rather provides some means by which their toxins are rendered 

 harmless. In accordance with our present conception of chemotaxis, it may 

 therefore be surmised that, in susceptible subjects, when the specific germs, 

 together with their toxins, are introduced, a number of leucocytes will 



