17.1 



of its brief sojourn in the human host except the Immunity which a first 

 attack of yellow fever confers. The person tlms immunized can thereafter 

 receive with impunity the hites of any number of infected stegomyiaa and 

 becomes incapacitated from contributing again to the perpetuation of the 

 germ. 



Since the first day of the attack of yellow fever (as early as the ninth 

 hour in one of Dr. Carroll's cases), any healthy stegomyia that hites the 

 patient is liable to become infected, so thai a new life eyele will he opened 

 for the germ in the newly infected stegomyia. Thus by alternately passing 

 from the human to the mosquito-host and vice versa, the yellow fever 

 germ has now lived during several consecutive centuries in tropical America, 

 where both the aborigines and the. European invaders belonged to 

 susceptible races. 



Now let us consider the probable character of the evolutions which 

 the yellow fever germ undergoes in the body of the human host. Bearing in 

 mind the fact that other twodiost protozoa, such as the malaria parasite, 

 lead during several consecutive months a parasitic existence, multiplying 

 by schizogonia, forming schizonts, merozoits and, after a while, also 

 gametes, in the body of their permanent host, and are unable to 

 accomplish the sexual reproduction indispensable for the perpetuation of 

 their species, unless they pass into the body of a second host (the Anopheles, 

 in the ease of the malaria parasite) ; bearing this in mind, are we not 

 justified in supposing that the yellow, fever parasite, being unable to 

 accomplish this sexual reproduction in the body of its permanent mosquito- 

 host, can only succeed in doing so by passing into the body of a non-immune 

 human being after some of its elements have reached a stage of sexual 

 differentiation corresponding to the gametes of malaria .' 



There is, moreover, another important consideration which may be 

 adduced in support of this interpretation. The large tertian parasite, 

 untreated, may continue for many months in the body of infected persons 

 without immediate danger to the patient, whereas Grassi, I am informed. 

 has found all the anopheles, in the Roman marshes, free from parasites 

 during the winter season, so that all the infected ones must have died 

 in consequence of their infection (corresponding to the phase of sexual 

 reproduction of the parasite), and he attributes the renewal of the malaria 

 epidemic in. the following spring to the survival of the parasites in the 

 human permanent host, in whose body only schizogonic phases arc 

 accomplished. This shows that the act of sexual reproduction is attended 

 with more dangerous consequences for the host in which it is accomplished 

 than are the various schizogonic phases of the same germ, a result which 

 I bad already surmised by reason of the greater metabolic changes which 

 must accompany the sexual reproduction as compared with the schizogonic 

 multiplication. 



If it be admitted, therefore, that the penetration of the germ into the 



