l86 YELLOW FEVER [CH. 



commencement of their infection. A young American doctor, 

 Dr Moran, entered this half of the house and was bitten by 

 a number of the mosquitoes. After a short incubation period, 

 he developed a typical attack of yellow fever, whilst persons 

 who occupied the other half of the house for at least 20 days 

 remained healthy. 



In this manner was established the important truth that 

 yellow fever is only carried by means of the mosquito, and the 

 above-mentioned results have been repeatedly confirmed by 

 subsequent investigators. 



The causal agent. In spite of numerous researches the 

 causal agent is still under discussion, for most of the organisms 

 that have been described from yellow fever patients may be 

 explained as the result of secondary infection, or in other cases, 

 as' artefacts. Because of the many points of resemblance be- 

 tween relapsing fever and yellow fever, it has been suggested 

 that the latter is also due to a spirochsete. Recently Seidelin 

 has described the occurrence of a parasite, belonging to the 

 Babesiidae, in the blood and organs of yellow fever patients. 

 To this parasite Seidelin has given the name of Paraplasma 

 flavigenum, and he considers it to be the causal agent of yellow 

 fever. 



Whatever the causal agent may be, it is present in the 

 circulation in large numbers, for the subcutaneous injection of 

 minute quantities of infected blood into a non-immune person 

 is followed by an attack of yellow fever. It is said to be 

 present in the circulation only during the first three days of 

 an attack, for blood taken from a patient on the fourth day, 

 when his temperature was stih high (40° C.) was found to be 

 non-infective. However, as there has only been one attempt 

 made to infect persons with the blood taken from patients 

 after the third day, this statement is still open to question. 

 On the whole it seems probable that yellow fever patients may 

 remain infective even after all signs of the disease have dis- 

 appeared, for it is difficult to explain its prevalence in certain 

 cases except on the theory of latent infections. The organism 

 must be excessively minute for it can easily pass through a 

 Chamberland Filter F. When the infected serum is diluted 



