444 



yellow fever patient being only such as might be expected of a parasite 

 going through phases of sexual reproduction in the body of its intermediate 

 host. 



In order to compare the two germs with each other, I shall recall some 

 of the most essential phases of development in the malaria parasite. The 

 new born embryos abandon the salivary glands of the anopheles to 

 commence their life cycle in the body of a non-immune who has been 

 bitten by the insect. There, in the body of the non-immune subject, the 

 young parasites will find appropriate conditions for their subsequent 

 development; they will grow to adult age, multiplying by schizogonia or 

 sporulation in the blood of their host, and will continue to do so for an 

 indefinite length of time even after they have become fit for sexual repro- 

 duction. The phenomena of sporulation being repeated at regular intervals, 

 a reaction is produced each time in the non-immune host in the form of 

 characteristic attacks of intermittent fever. The tenacity with which these 

 fevers take possession of the patients, lasting sometimes, if not properly 

 treated, several consecutive months, clearly proves that the human body 

 constitutes the permanent host for the parasite. Yet within the body of this 

 permanent host, for some unknown reason (perhaps the need of lower tem- 

 peratures or of a freer access of atmospheric air), the function of sexual 

 reproduction for the malaria parasite con not be even initiated, and it 

 can only be accomplished in all its phases within the body of an 

 intermediary host, the anopheles mosquito. If the anopheles tarries too 

 long, resting bodies (crescents) are formed, in which condition the 

 parasites can abide almost indefinitely in the blood of the infected malaria 

 subject, without occasioning any outward symptoms, but ever ready to 

 assume the function of sexual reproduction whenever an anopheles 

 mosquito happens to suck them up with the blood in the act of biting the 

 patient. The anopheles will then act the part of an intermediary host in 

 whose body, after the lapse of a few days, a large number of embryo 

 parasites will be produced. These, by virtue of some occult biological 

 affinity, accumulate in the salivary glands of the intermediate host, so 

 that the latter may implant them into a capilary vessel of a non-immune. 

 Should an appropriated host, however, fail to come within reach of the 

 anopheles : sting, after the new generation of parasites have gathered in 

 its salivary glands, they will die either without parting from the insect 

 or upon inappropiate soils where they may have been deposited by the 

 latter with its saliva. The intermediate host appears therefore to constitute 

 a special device provided by nature to prevent the extinction of the 

 parasite. 



Now, if we compare the facts that are known regarding the clinical 

 history, etiology, and epidemiology of yellow fever with the foregoing 

 account, what strikes one the most is' not the paralellism but rather the 

 contrast between the course of events in malaria and in yellow fever, as 



