INFECTIOUS DISEASES. 301 



of seaport cities in tropical and semitropical regions, but in these 

 regions its prevalence is greatly restricted. In North America, 

 although it has occasionally prevailed as an epidemic in every one of 

 our seaport cities as far north as Boston, and in the Mississippi Valley 

 as far north as St. Louis, it has not established itself as an endemic 

 disease within the limits of the United States. In South America 

 it has prevailed as an epidemic at all of the seaports on the Gulf, and 

 on the Atlantic coast as far south as Montevideo and Buenos Ayres ; 

 also at several seaports of Mexico and Peru on the Pacific. At 

 present the principal endemic foci of the disease are Havana, Vera 

 Cruz, and Rio Janeiro. In Africa the disease is limited to the west 

 coast, and so far as we know no epidemics have prevailed in the 

 interior of tropical Africa, although the conditions would appear to 

 be favorable for the development of an epidemic in case the disease 

 should be introduced. The same is true as regards the populous 

 regions in northern Africa and southern Asia where the rainfall is 

 sufficient. The disease does not prevail in arid regions, or at con- 

 siderable elevations above the sea level. 



Yellow fever does not prevail as an endemic disease in places 

 which have a mean winter temperature much below 65° P. (18.3° 

 C), and as a rule epidemics are not developed at a lower temperature 

 than 75° to 80° P. (23.8° to 26.6° C). The approach of cool 

 weather checks the progress of an epidemic, and in those endemic 

 foci of the disease (Havana, Rio de Janeiro, Vera Cruz) where it pre- 

 vails annually it is essentially a disease of the summer months. That 

 moisture is an essential factor is indicated by the fact that the disease 

 does not prevail in arid regions where other conditions appear to be 

 favorable, and that it is especially a disease of the seacoast and of the 

 margins of great rivers. Heavy rains, however, exercise a favorable 

 influence in checking an epidemic — probably by cleansing the 

 streets, sewers, etc., in an infected locality. In the tropics the 

 commencement of the rainy season often puts an end to the prevail- 

 ing epidemic. 



Decomposing matter of animal origin appears to form a favor- 

 able nidus for the development of the hypothetical yellow-fever 

 germ. It is a disease of towns and cities, and especially of such as 

 are in an insanitary condition. The writer's studies have led him to 

 the conclusion that the infectious agent, as in cholera and in typhoid 

 fever, is probably present in the discharges of the sick. 



The early history of yellow fever is involved in obscurity, and it 

 is doubtful whether we will ever be able to settle in a definite manner 

 the disputed question as to its origin. Two principal theories have 

 been advanced: one that it was endemic at certain points on the 

 shores of the Gulf of Mexico at the time of the discovery of the New 



