It is my persona] opinion that three conditions are necessary in order 

 that the propagation of yellow fever shall take place: 



1. The presence of a previous case of yellow lever within certain 

 limits of time, counting from the moment that we are considering. 



2. The presence of a person apt to contract the disease. 



3. The presence of an agent entirely independent for its existence 

 both of the disease and of the sick man, but which is necessary in order 

 that the disiase shall be conveyed from the yellow fever patient to a 

 healthy individual. 



It will be objected that this is a mere hypothesis: and, indeed, is is only 

 as such that 1 give it. Hut 1 believe it is a plausible one. which has, at 

 least, the merit of explaining a certain number of facts which have remain- 

 id hitherto unaccounted for by the current theories. I do not ask for any- 

 thing else, as my only object is to show that, if my hypothesis, or some other 

 analogous to it. should he realized, all the measures which are now em- 

 ployed in order to disinfect and to check the progress of the disease would 

 turn out to he withohut effect, in as much as the principal efforts should 

 have been directed against the third condition, by endeavoring to destroy the 

 agent of transmission or to divert it from the path that it follows in 

 communicating the disease. 



You see. therefore, gentlemen, how important it is that this question 

 should he thoroughly studied if we do not wish to be led upon a false track 

 while recommending particular measures against the propagation of 

 the disease. 



