136 



A number of towns, between whicb there is intercommunication, 

 may represent a permanent focus of yellow fever, though perhaps no 

 single one could continue as such by itself. In such an area, the 

 rapidity of spread of the fever, and hence the exhaustion of the sus- 

 ceptible material, would be much slower than in a single town. The 

 effect of such a permanent focus is not to prevent other communities 

 from freeing themselves from yellow fever by the process outlined 

 above, but to re-infect them as soon as a sufficient number of susceptible 

 individuals has accumulated. It is this recurrence of fever which has 

 given rise to the belief in " larval " fever held by many authorities. 

 Yellow fever undoubtedly does sometimes exist unrecognised among 

 the native children of a community, being noticed only when it attacks 

 a stranger. In this way there is true recrudescence whenever an influx 

 of strangers occurs, this being quite different from the re-infections 

 that occur by reason of adult immigration. In order, therefore, that 

 a town which has freed itself from yellow fever by the failure of the 

 human host may remain permanently free from fever, isolation from 

 infected places is necessary. When yellow fever has been eliminated 

 by the control of the insect host, this isolation is not necessary as long 

 as this control continues to be efficient, because, in the absence of the 

 mosquito, yellow fever is not communicable, and such parasites as 

 might be 'brought in by infected persons or infected mosquitos would, 

 at the most, establish a very temporary focus of infection. If the con- 

 trol were complete, men infected by mosquitos would transmit no 

 parasites to other mosquitos, and there would be no secondary human 

 cases, the possibility of conveyance of the disease ending with the 

 death of the mosquitos introduced. Naturally, the method for the 

 elimination of yellow fever by control of the insect host would be that 

 chosen by the sanitarian, while in nature the method is by failure 

 of the human host. 



These deductions can be applied to explain the great dimmution of 

 yellow fever in the Americas. In this way the great permanent foci 

 of Havana, Vera Cruz, Panama and Rio were extinguished and with 

 them a number of smaller places were freed by control of the insect 

 host. Steam vessels built of iron very rarely carry Stegomyia, as the 

 old sailing vessels frequently did, and hence are much less efficient 

 in the transport of parasites. The European war has restricted foreign 

 immigration and greatly lessened the movement of people between 

 different towns. Thus places which have cleared themselves of infection 

 are far less likely to be re-infected ; so long as the parasite is absent, even 

 an influx of susceptible people has brought no recrudesence of fever. 



These facts indicate that a well-organised effort against yellow fever 

 will result in its complete elimination from the earth, so that it will 

 never return. The opportunity for making such an effort has never 

 been so favourable as at the present time. It is upon these facts that 

 the recommendations of the Yellow Fever Commission of the Inter- 

 national Health Commission have been based, their purpose being the 

 permanent elimination of yellow fever from the globe. This will be 

 the first time in history that an attempt is made to eliminate com- 

 pletely a micro-organism pathogenic to man ; its accomphshment 

 will mark an epoch in sanitation. 



A long and interesting discussion, which followed the readmg of this 

 paper, is given verbatim. 



