HYGIENE 



539 



The sun's ra}'s will cause congestion of the skin, followed by 

 increased pig-mentation, sunburn, dermatitis, and probablv xeroderma 

 pigmentation. 



Heat exhaustion is perhaps due directly to meteorological con- 

 ditions, but sunstroke and siriasis are not so easy to explain. For 

 details of these two disorders see separate section. 



Neurasthenia is almost universal. 



Loss of memory is common, especially on the West Coast of 

 Africa, where it is known as " Coast Memory." 



THE RELATION OF CLIMATE AND HYGIENE TO TROPICAL 



DISEASES. 



A few of the outstanding features onlv will here be dealt with as 

 these diseases are described elsewhere. 



Asiatic Cholera. 



This disease often subsides or disappears in the winter and in 

 temperate climates, but it reappears witli the warm weather without 

 fresh introduction; hence heat is an important predisposing cause of 

 the disease. Heat is not all, however, for if the temperature is rising, 

 as in Calcutta during April and May, and the rainy season sets in, 

 cholera is diminished, but when the rains cease in November it again 

 increases, for at this time the temperature again falls. A moist soil, 

 prolonged heat and drought are the best conditions for the spread of 

 the disease. 



It is a water-borne disease; hence no water should be allowed to 

 become polluted, and all water should be sterilized before drinking. 



Insanitary conditions of air, water, food, soil, drainage and housing 

 determine the point of attack. 



Yellow Fever. 



This disease is endemic only where the mean temperature reaches 

 68° F. (20° C). Frost always arrests it. I'nless the temperature is 

 very low, atmospheric humiditv and rainfall favour it. 



It is limited to the sea coasts and the mouths of great rivers. 



It is seldom found over 700 feet high. 



The disease commences usually in the most crowded and filthiest 

 quarter of the town or ship if similar conditions prevail. 



As the ultra-microscopic virus is mosquito-borne (Stegomyia 

 calopus), all conditions favourable for mosquito breeding are favour- 

 able for the spread of the disease, e.g., no mosquitoes, no yellow 

 fever. 



Plague. 



This is an infectious disease of filtli that has robbed India alone 

 of more than 7,000,000 of its inhabitants. It is fostered by filth, 

 overcrowding, lack of ventilation and destitution. 



