162 MALARIA 



is entirely absent. It is suggested by Manson that the fever is 

 caused by a distinct organism, and that malaria is merely a 

 predisposing cause. 



Immunity and Epidemics. — Absolute immunity to malaria 

 is rarely if ever acquired but, as already remarked, oft-repeated 

 infections especially in childhood tend to build up a high de- 

 gree of tolerance to the effects of the parasites and a diminution 

 in the number of parasites in the body. The protection afforded 

 by a single infection is very shght, and is retained for only a 

 short time in the absence of reinfections. Even the cumula- 

 tive effect of numerous infections disappears rapidly in the 

 course of a few years. Some authors divide malaria into two 

 types. There is a " tropical " form, occurring in places where 

 reinfections can occur practically throughout the year on 

 account of the continued warm temperatiu'e. The other, a 

 " subtropical " form, is found in regions where cold weather 

 causes an annual seasonal interruption of infection by a cessation 

 of breeding on the part of Anopheles, and by a discontinuance of 

 growth on the part of the parasites in the mosquitoes. In tropi- 

 cal malaria a fairly constant degree of immunity is maintained, 

 and epidemics are rare if they occur at all. In Java and other 

 tropical places, according to Robert Koch, nearly every native 

 child, under four years of age, has his blood teeming with ma- 

 laria parasites from which he suffers little inconvenience. These 

 parasites gradually become scarcer in older children and are 

 often practically absent in adults who, however, have been shown 

 to be passive " carriers " of small numbers of the parasites and 

 therefore a source of danger to the community. The " carriers," 

 though relatively immune to the more acute symptoms of the 

 disease, are left in the run-down condition of malarial cachexia. 

 As pointed out by Gill, there is a striking analogy between the 

 confirmed opium-eater and the malarial cachectic. Both have 

 purchased their immunity at a heavy price. In the former the 

 emaciated frame, sallow complexion and other signs of debility 

 proclaim the victim of a drug habit; in the latter the enlarged 

 spleen, the lack of physical and mental energy, and the shrunken 

 body bear witness to the havoc wrought by long-standing ma- 

 laria. In the case of neither does death often take place as the^ 

 direct effect of their respective poisons, but both readily fall 

 victims to intercurrent affections. In subtropical malaria, on 



