PHENOMENA OF INFECTION. 89 



acclimated and survive the cold of the winter months, 

 breed fresh generations the following summer, and in 

 this way establish a new endemic focus for the diseases 

 which they carry. 



A good example of an insect which acts as host for 

 an infectious animal micro-organism, and one which 

 depends for existence upon the physical conditions of a 

 country, is found in the species of mosquito (anopheles) 

 which transmits malarial fever. As the anopheles 

 only breeds in shallow puddles of water or slowly 

 moving streams, it is manifest that w^ll-drained regions 

 are likely to be free from malaria, while those presenting 

 an opposite condition are favorable to its introduction. 

 This same species of mosquito neither breeds nor bites 

 when the temperature is below 68 Fahrenheit, a fact 

 which explains the constant presence of ague in tropical 

 climates. High altitudes, and mountainous regions, 

 because they do not present the physical conditions 

 necessary for the multiplication of the anopheles, are 

 free from malarial fever; but the same is not true for 

 yellow fever, which is transmitted by another species 

 of mosquito, the stegomyia fasciata. This species 

 breeds in rain-spouts, cisterns, and small collections 

 of water about a household. Given a favoring climate, 

 therefore, where the practice of storing water about 

 dwellings prevails, there is no reason why this pestif- 

 erous insect cannot become domesticated in regions 

 that are now outside the yellow fever zones. An 



