TRANSMISSION OF YELLOW FEVER BY MOSCiUITOES. 671 



ing, bedding, etc., there seems no o-ood reason why it sliould not have 

 Vjeen carried to these distant localities long- ago. 



The restriction as regards altitude, however, probabh^ 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 seaport, Vera Cruz. Persons who have been exposed 

 in Vera Cruz during the epidemic season frequenth' fall sick after 

 their arrival in the City of Mexico, ]>ut they do not communicate the 

 disease to those in attendance 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 I)edding and 

 the insanitary local conditions 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 Oidex fascldfus in 

 high altitudes, but the inference that it is not to be found in such 

 localities as the Cit}' 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 rarel}" 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 74.5 feet above sea level. 



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

 result of inoculation by mosquitoes of the genus Anopheles, 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 resem- 

 bled that of the malarial fevers it would no doubt have been discov- 

 ered long ago, but as a matter of fact this parasite, which we now 

 know is present in the blood of those sick with the disease, has thus 

 far eluded all researches. Possibly it is ultramiscroscopic. How- 

 ever this may be, it is not the only infectious-disease germ which 

 remains to be discovered. There is without doubt a living germ in 

 vaccine l3'mph and in the virus from smallpox pustules, but it has not 

 been demonstrated by the microscope. The same is true of foot-and- 

 mouth disease and of infectious pleuro-pneumonia of cattle, although 

 we know that a living element of some kind is present in the infec- 

 tious material by which these diseases are propagated. In Texas 

 fever of cattle, which is transmitted by infected ticks, the parasite is 

 very minute, but by proper staining methods and a good miscroscope 

 it may l)e detected in the interior of the red blood corpuscles. Doc- 

 tors Reed and Carroll are at present engaged in a search for the 



