MALARIA 71 



(3) Occupation in deltas of rivers, low-lying marshy districts, or 



working in such places as rice fields, recently deforested 

 lands, &c. 



(4) Age. Infants are more readily attacked than adults. 



(5) Racial. Native races suffer less than immigrant races in an 



infected country because of the partial immunity of 

 indigenous peoples. 



(6) Diminished bodily resistance and the factors that cause it, 



e.g., chills, starvation, over-feeding, the onset of other 

 diseases, chronic ailments, &c. 



(7) Altitude. Mosquitoes do not thrive at high altitudes in the 



tropics due to the cool air and winds present there. ]^Ialarial 

 ■ "' convalescents should be sent to the hills, but at slow stages. 



(8) Decomposing vegetable matter. This is not indispensable, 



but it is a stimulating factor for fly breeding when present. 



(9) Subsoil moisture. Where the subsoil moisture frequently 



rises high, there facilities are afforded for fly breeding, 

 given other favourable conditions. 



(10) Rainfall. Heavy rains wash out the drains, pools, and with 



them the contained larvae. Less heavy rains fill them only. 

 Where water lies for several weeks in the tropics mosquito 

 breeding is stimulated. Where there is no water, as in the 

 Sahara desert, there is no malaria, the oasis excepted. 



(11) vStrong winds. Mosquitoes fly a few feet from the ground. 



When wind or currents of air prevail, as from fans and 

 punkahs, mosquitoes seek shelter. 



(12) Trees. A belt of trees between breeding places and dwellings 



is said to be j^rotective. 



(13) Time of day. Mosquitoes bite most frequently just before 



sunrise and just after sunset and at night. In shady or dull 

 places they will bite at any time. There are stragglers at 

 all hours. 



(14) Disturbance of soil. Tliis predisposes in so far as there are 



usually excavations in which puddles of water accumulate 

 for mosquito breeding. 



In construction of roads, railwavs, houses, forts, barracks, 

 &c., carelessness in this respect may cost the enterprise 

 hundreds of lives, as happened at Hongkong. 

 Exciting causes. 



The exciting causes are P. malarici?, P. vivax, and L. malariae 

 because (Manson) : — 



(i) Their occurrence in the blood is sooner or later associated 

 with the malarial clinical phenomena. 



