228 POPULAR SCIENCE MONTHLY. 



disease ; also the fact that the epidemic extension of the disease depends 

 upon external conditions relating to temperature, altitude, rainfall, etc. 

 It was a well-establislied fact that the disease is arrested by cold weather 

 and does not prevail in northern latitudes or at considerable altitudes. 

 But diseases which are directly transmitted from man to man by per- 

 sonal contact have no such limitations. The alternate theory took ac- 

 count of the above-mentioned facts and assumed that the disease was 

 indirectly transmitted from the sick to the well, as is the case in typhoid 

 fever and cholera, and that its germ was capable of development ex- 

 ternal to the human body when conditions were favorable. These con- 

 ditions were believed to be a certain elevation of temj)erature, the pres- 

 ence of moisture and suitable organic pabulum (filth) for the develop- 

 ment of the germ. The two first-mentioned conditions were known to 

 be essential, the third was a subject of controversy. 



Yellow fever epidemics do not occur in the winter months in the 

 temperate zone and they do not occur in arid regions. As epidemics 

 have frequently prevailed in sea-coast cities known to be in an insani- 

 tary condition, it has been generally assumed that the presence of de- 

 composing organic material is favorable for the development of an epi- 

 demic and that, like typhoid fever and cholera, yellow fever is a 'filth 

 disease.' Opposed to this view, however, is the fact that epidemics have 

 frequently occurred in localities {e. g., at military posts) where no local 

 insanitary conditions were to be found. Moreover, there are marked 

 differences in regard to the transmission of the recognized filth diseases 

 — typhoid fever and cholera — and yellow fever. The first-mentioned 

 diseases are largely propagated by means of a contaminated water 

 supply, whereas there is no evidence that yellow fever is ever communi- 

 cated in this way. Typhoid fever and cholera prevail in all parts of the 

 world and may prevail at any season of the year, although cholera, as a 

 rule, is a disease of the summer months. On the other hand, yellow 

 fever has a very restricted area of prevalence and is essentially a disease 

 of seaboard cities and of warm climates. Evidently neither of the 

 theories referred to accounts for all of the observed facts with reference 

 to the endemic prevalence and epidemic extension of the disease under 

 consideration. 



Having for years given much thought to this subject, I became some 

 time since impressed with the view that probably in yellow fever, as in 

 the malarial fevers, there is an 'intermediate host.' I therefore sug- 

 gested to Dr. Eeed, president of the board* appointed upon my recom- 

 mendation for the study of this disease in the Island of Cuba, that he 

 should give special attention to the possibility of transmission by some 



* The members of the board were : Major Walter Reed, Surgeon U. S. A. ; 

 Dr. James Carroll, Contract Surgeon U. S. A. ; Dr. A. Agramonte, Contract Sur- 

 geon U. S. A., and Dr. Jesse W. Lazear, Contract Surgeon U. S. A. 



