124 TROPICAL MEDICINE AND HYGIENE 



in an endemic mild form, and that highly susceptible 

 new-comers may acquire the disease in a virulent form 

 from the mosquitoes infected from the resident popula- 

 tion. Such virulent cases would be the only ones to 

 attract attention and would be considered as sporadic 

 cases. The commoner mild cases would either be over- 

 looked altogether or grouped with bilious remittent fever. 



Simond, Aubert, and Noe consider that in Martinique 

 the epidemics which occur from time to time are acute 

 manifestations of the endemic disorder. The variations 

 in the severity of yellow fever they explain either as the 

 result of an exceptionally large dose inoculated by the 

 mosquito, or and this is more probable that the virulence 

 of the organism is increased by passage through a series 

 of susceptible adults and diminished by passage through a 

 series of partially immune adults or children, such as must 

 be present in a country where the disease is endemic. 

 This would explain why when yellow fever occurs on 

 board a ship manned by Europeans or amongst the 

 members of a European regiment crowded in barracks, 

 where the infection after a time will usually be derived 

 from another susceptible person, the mortality is very 

 high, 50 per cent, or more, whilst in the later epidemics, as 

 in that described by Blair in 1843, in British Guiana, the 

 mortality was only 13-3 per cent, in the whole epidemic 

 and nil in the mild form then first described. In the first 

 general outbreak throughout the West Indies the mor- 

 tality amongst all races was very high during the first year 

 of the invasion, one to two-thirds of the cases died, though 

 later a larger proportion recovered. 



Manson suggests a further hypothesis, that possibly 

 man is not the only reservoir, and that some of the lower 

 animals may receive and carry on the infection in a mild 

 form. We know that anthropoid apes are susceptible, but 

 as regards other animals, if susceptible, the disease does 

 not appear to affect their health. If a lower animal 

 serves as the reservoir that animal would probably be a 

 rat, as the early distribution of the cases is in the 



