Distribution of Tropical Diseases in Africa. 425 
decreasing elevations, and therefore heat, atmospheric pres- 
sure, etc., vary at different latitudes. 
The influence of these zones upon disease is, broadly 
speaking, as follows:—In the upper or cold zone there is a 
tendency to plethora; the disorders met with are of an 
inflammatory character, and the diseases of the respiratory 
and circulatory organs are far from uncommon. Malaria, 
yellow fever, cholera, phthisis, dysentery, and hepatitis are 
almost entirely absent. The absence of phthisis is probably 
due to the rarification of the atmosphere, the absence of 
yellow fever to the lower temperature, as well as on account 
of the distance from the sea, 
Areas situated at these high altitudes are incomparably 
superior to the low-lying districts. As a rule they are 
entirely free from tropical endemic diseases, which, should 
they perchance be introduced, rarely spread. A marked 
difference is soon noticed in the appearance of individuals 
who, after a residence in low-lying tropical countries, go to 
the hills or elevated tablelands. Their vigour improves 
rapidly, they regain almost the healthy appearance they had 
in Europe, their digestion and the composition of their blood 
is improved, and a proper amount of exercise can be taken. 
“ill diarrhea” and rheumatism may sometimes affect 
them in these higher altitudes. The former may be due to 
either a faulty water-supply or a too rapid removal from the 
plains; the latter—as in India—to damp dwellings, or to 
the more marked vicissitudes of climate. 
The middle zone is perhaps best divided into two, which 
we may compare with the temperate and sub-temperate 
zones, the cooler (higher) having a mean annual temperature 
of from 41° F. to 55° F., the warmer (lower) a mean annual 
temperature of from 55° to 73° F. In both these areas the 
seasons exert an influence, and therefore the presence and 
prevalence of disease fluctuate, the frequency of diseases of 
the respiratory and digestive organs, for example, rising and 
falling correspondingly; but throughout this region, as a 
whole, diseases specially frequent in the higher and lower 
zones are less virulent in character. 
The lowest or hot zone is the typical tropical disease zone. 
