78 TROPICAL MEDICINE AND HYGIENE 



place where he was bitten by an infective anopheline. 

 That mosquito must have fed on a person in whose 

 blood gametocytes were present some ten days before. 

 It must always be remembered that the gametocytes are 

 present in a person convalescent from a malarial attack, 

 especially from subtertian "malaria. 



In a country where malaria is prevalent, those liable 

 to attack may be divided into two main classes, both 

 consisting of persons who have not acquired immunity 

 by previous attacks of malaria : 



(1) Children, native and European, will be liable to 

 harbour the parasites, as there is no hereditary immunity. 

 There may be some hereditary tolerance. Adults who 

 have lived for a long time in the country will have 

 acquired some immunity and therefore will not com- 

 monly harbour parasites. 



(2) Susceptible adults, European and native, are those 

 who have lived long in a non-malarial country, and are 

 therefore not immune. Either of these classes in a 

 malarial country may serve as the starting-point of fresh 

 cases by infecting the mosquitoes. 



The first class are of most importance in fixed settle- 

 ments and also in infecting travellers passing through 

 a country. 



The second class are of special importance in opening 

 up uninhabited country ; in mines, plantations, and road 

 or railway construction. 



With large bodies of men outbreaks may originate 

 from infection acquired from native children, or may be 

 imported by some of the adults joining such a force, as 

 these may harbour the parasites. When malaria is once 

 introduced amongst such a body of men, each case serves 

 to infect the mosquitoes, and these in turn infect other 

 men till all susceptible members of the gang are attacked. 



In travelling through a malarial country an endeavour 

 should be made to avoid being bitten by mosquitoes by 

 the free use of mosquito netting, by avoidance of places, 

 particularly for camps, where the mosquitoes are likely 



