562 ANNUAL EEPORT SMITHSONIAN INSTITUTION, 19 30 



moisture, is of fundamental importance in many cases. As the late 

 Sir Patrick Manson pointed out, many years ago, in his famous work 

 on tropical diseases, the term " tropical diseases " does not mean 

 diseases confined to the Tropics. It was employed by him in a meteor- 

 ological sense for diseases associated with, but not solely or even di- 

 rectly due to high temperatures. Tropical climatic conditions, per se, 

 it may be noted in passing, probably do not injuriously affect the 

 natives of the Tropics any more than do the conditions of extratrop- 

 ical climates affect us who live in them. Most tropical diseases attack 

 both natives and whites; sometimes the former suffer most; some- 

 times the latter. There is no rigid hard-and-fast rule. The racial 

 element is, however, often potent. Finally, great numbers of tropical 

 natives have inevitably, in the long succession of generations, become 

 more or less immune to the attacks of disease against which the whites 

 have not yet developed immunity. 



Leaving these broad generalizations, let us turn next to a brief 

 consideration of a few of the more important tropical diseases. In 

 addition to the more or less direct effects of exposure to tropical sun- 

 sliine, dampness and heat, such as sunstroke, heat exhaustion and the 

 like, there are malaria, and dysentery in various forms, doubtless the 

 two worst enemies of white residents in the Tropics; sleeping sick- 

 ness ; tropical abscess of the liver ; hookworm and other intestinal par- 

 asitical infectious fevers of different kinds ; ulcers and other infections 

 of the skin; many infectious diseases common to colder as well as 

 warmer latitudes, and other ailments that chiefly attack the natives, 

 and are therefore of a medical rather than of a practical interest in 

 our present consideration. Yellow fever, cholera, and plague are 

 very greatly limited, and are being more and more successfully over- 

 come as the result of modern sanitary measures. 



Twenty years ago Sir Ronald Ross wrote of malaria, " I venture to 

 say that it has profoundly modified the history of mankind by doing 

 more than anything else to hamper the work of civilization in the 

 Tropics," During the two decades that have elapsed since that state- 

 ment was made, the campaign against malaria has accomplished 

 remarkable results, both in prevention and in treatment ; yet it is 

 safe to say that malaria still remains one of the most destructive 

 diseases in the world. Rough estimates — the only ones that are pos- 

 sible at present — indicate that about one-third of the population 

 throughout the Tropics suffers from it constantly. The malarial zone 

 is a broad band within the moist Tropics, extending at times into the 

 margins of the adjoining temperate zones. The disease is absent, or 

 infrequent, in deserts and steppes, and on mountains at moderate 

 elevations above sea level. It is perennial in its zone of maximum 

 frequency, but has a general tendency to rise to a peak in the warmer 

 or rainy season. Even in the early days of medical investigation into 



