YELLOW FEVER AND MOSQUITOES. 239 



yellow fever prevails. The propagation of the disease depends upon the 

 introduction of an infected individual to a locality where this mos- 

 quito is found, at a season of the year when it is active. Owing to the 

 short period of incubation (five days or less), the brief duration of the 

 disease and especially of the period during which the infectious agent 

 (germ) is found in the blood, it is evident that ships sailing from in- 

 fected ports, upon which cases of yellow fever develop, are not likely to 

 introduce the disease to distant seaports. The continuance of an 

 epidemic on ship-board, as on the land, must depend upon the presence 

 of infected mosquitoes and of non-immune individuals. Under these 

 conditions we can readily understand why the disease should not be car- 

 ried from the West Indies or from South America to the Mediterranean, 

 to the east coast of Africa or to Asiatic seaport cities. On the other 

 hand, if the disease could be transmitted by infected clothing, bedding, 

 etc., there seems no good reason why it should not have been carried to 

 these distant localities long ago. 



The restriction as regards altitude, however, probably depends upon 

 the fact that the mosquito which serves as an intermediate host is a coast 

 species, which does not live in elevated regions. It is a well-established 

 fact that yellow fever has never prevailed in the City of Mexico, although 

 this city has constant and unrestricted intercourse with the infected sea- 

 port. Vera Cruz. Persons who have been exposed in Vera Cruz during 

 the epidemic season frequently fall sick after their arrival in the City 

 of Mexico, but they do not communicate the disease to those in attend- 

 ance upon them or to others in the vicinity. Evidently some factor 

 essential for the propagation of the disease is absent, although we have 

 the sick man, his clothing and bedding and the insanitary local condi- 

 tions which have been supposed to constitute an essential factor. I am 

 not aware that any observations have been made with reference to the 

 presence or absence of Culex fasciatus in high altitudes, but the infer- 

 ence that it is not to be foimd in such localities as the City of Mexico 

 seems justified by the established facts already referred to. 



As pointed out by Hirsch, "the disease stops short at many points in 

 the West Indies where the climate is still in the highest degree tropical." 

 In the Antilles it has rarely appeared at a height of more than 700 feet. 

 In the United States the most elevated locality in which the disease has 

 prevailed as an epidemic is Chattanooga, Tenn., which is 745 feet above 

 sea level. 



It will be remembered that the malarial fevers are contracted as a 

 result of inoculation by mosquitoes of the genus Anoplieles, and that 

 the malarial parasite has been demonstrated not only in the blood of 

 those suffering from malarial infection, but also in the stomach and 

 salivary glands of the mosquito. If the yellow fever parasite resembled 

 that of the malarial fevers it would no doubt have been discovered long 



