Yellow Fever Immunity. Modes of Propagation. 

 Mosquito Theory 1 ) 



Observers mostly agree in considering Yellow Fever as a germ disease, 

 although no satisfactory demonstration of its specific germ has yet been 

 produced. This deficiency is to be deplored all the more as certain peculiar- 

 ities connected with its development and propagation appear to single 

 it out from among other infections and suggest the necessity for special 

 protective measures. 



Yellow fever is a specific, transmissible disease, running an acute 

 course and peculiar to certain localities along the American and African 

 coasts of the Atlantic Ocean, between the tropics. It may indeed be prop- 

 agated through ships or overland to other regions occupying low levels and 

 having, for the time being, a mean temperature above 18° C. ; but it seems 

 incapable of obtaining a permanent foothold outside of the tropics and be- 

 yond certain altitudinal limits. 



When a considerable number of susceptible subjects live scattered 

 among the rest of the population in a locality like Havana, where Yellow 

 Fever is endemic, some may be attacked within a few days of their arrival, 

 whilst others may continue to reside during consecutive weeks, months or 

 years, without experiencing the slightest indisposition that could be attri- 

 buted to Yellow Fever infection. The length of this period of exemption 

 depends in a great measure on the chances they have had of entering the 

 area of infection surrounding each individual case of the disease; an area 

 which appears to be at all times of limited extent but much more so during 

 what is called the "non-epidemic" season (October to April), than during 

 the hotter months, from May to September. The same amount of exposure 

 does not affect indiscriminately or in equal degree, for, in some instances, 

 a quarter of an hour's stay in the sick room or in an infected house will 

 figure as the most probable source of the infection, whereas others may 



1) Comptes Bendus et Mémoires du Huitiéme Congrés International d'Hygiéne et 

 de Démographie, tenu á Budapest du 1 au 9 Septembre 1894, pages 702-706 (Bevised 

 by the author). 



