68 INFECTIOy A\D 



other piroplasmoses, and in malaria. In some in- 

 stances it is the micro-organism which is unknown 

 (yellow fever, dengue), in others the question of 

 inheritance in the insect (trypanosomiasis), in 

 others the relation of the insect and the disease to 

 hosts other than man. etc. 



Diseases which are transmitted habitually, or 

 insects, mainly, by insects sometimes possess rather dis- 

 tinctive epidemiologic features. Malaria occurs in 

 swampy regions in which certain species of Ano- 

 pheles abound. The distribution of yellow fever, 

 and indeed of all the insect-borne diseases, coin- 

 cides with that of the insects which are concerned 

 in the distribution. In the temperate and subtrop- 

 ical countries such diseases tend to prevail in the 

 warmer months and to disappear on the advent 

 of frost or cooler weather, an event which is cor- 

 related with the activity or inactivity of the insects 

 during these seasons. The Indian plague commis- 

 sion finds that an epidemic wanes when the mean 

 daily temperature is below 50 F., presumably be- 

 cause the flea does not become infected so readily 

 under this condition : the rats die before the advent 

 of intense septicemia, and without the latter the 

 flea is less likely to become infected. The flea 

 may also be less likely to feed generously in the 

 cooler weather. Also it was found that a mean 

 daily temperature of 85-90 F. coincides with the 

 decadence of an epidemic, and in harmony with 

 this it appears that the flea remains infective for 

 a much shorter time at this temperature. When 

 virtually all the rats of a locality have either- 

 been killed by plague, or have recovered from it, 

 those which remain are for the most part 

 immune, and the conditions for a recrudescence 



