Tropical Medicine and Hygiene 



CHAPTER I. 

 INTRODUCTORY. 



THE term Tropical Diseases is a convenient one, though 

 not capable of logical definition. Few diseases are limited 

 to the Tropics or even subtropical regions. As employed 

 in this work, it is meant to include all diseases which are 

 not commonly seen or recognized in England but which 

 are prevalent in tropical regions, and a few other 

 diseases which present peculiar characteristics, or require 

 special prophylactic measures in the Tropics. 



The peculiar distribution of many of the diseases met 

 with in the Tropics is due to the fact that the parasites 

 causing them require special conditions for their extra- 

 corporeal existence. These conditions in the case of 

 parasites such as ankylostomes, which do not require 

 an intermediate host, are mainly warmth and moisture. 

 Where, however, the parasites, like those of malaria, 

 require an alternative host for their development, the 

 conditions determining the distribution of disease are not 

 purely meteorological, but include various other factors 

 affecting the life of alternative hosts in that instance, 

 certain species of mosquitoes. These other factors include 

 the presence or absence of special soil, of water suitable 

 for breeding places, of suitable food for larvae, and so 

 forth. The absence of natural enemies of larvae or adult 

 forms of the insect hosts has also to be considered. 



The distribution of such alternative hosts and, there- 

 fore, of their parasites and the diseases caused by them, 

 has a great tendency to be local and apparently erratic, 



