366 
MALARIA 
transmission season, malaria covers the ap¬ 
proximate area it covered a centnry ago. 
There is no record of its natural recession 
from infected territory. On the contrary, 
the extension of industry, with its very low 
wage-scale has produced in some places a 
population of high density, inadequately 
fed and housed and poorly protected 
against disease. A'griculture in the tropics, 
in developing irrigation, has added greatly 
to the malarial burden, for control has been 
confined to the larger cities and to a few of 
the industrial settlements and to a very few 
of the larger agricultural projects. 
From Mexico to Panama and in the 
islands the intensity of malaria varies with 
the altitudes, with the vector species and 
with the density of the population. On the 
coastal plains it tends to be of moderate 
endemic intensity with frequent epidemics, 
though in portions of Mexico and British 
Honduras high endemic rates have been 
found. In the highlands, it is moderately 
endemic with scattered epidemics up to ele¬ 
vations of five thousand feet and in the very 
high mountain valleys it tends to be a low 
grade endemic with severe epidemics during 
the occasional very hot summers. It is 
hyperendemic in many of the interior val¬ 
leys at low elevations, particularly in south¬ 
ern Mexico. In the United States, it is of 
moderate endemic intensity in the sixteen 
southeastern states, with brief periods of 
high endemicity accompanied by scattered 
epidemics at intervals of approximately 
seven years, as shown in Chart I. This 
chart is a schematic representation of the 
estimated number of cases each year from 
CHAHT - 1 
Chart I. Trend of Malaria in the United States. 
