First, a temperature which does not fall below 20 deg.C.
(67.5 deg.F.); next, a very moderate degree of permanent humidity of the soil;
and finally, the direct action of the oxygen of the air upon the strata of
earth which contain the ferment. If a single one of these three conditions
be wanting, the development of malaria becomes impossible. This is a point
of prime importance in the natural history of malaria, and it gives us the
key to most of the methods of sanitary improvement attempted by man.
Let us see first what can be done in this direction without the labor of
man. For nature herself makes localities salubrious by _suspending_ for a
greater or less time the production of malaria. It is thus that winter
brings about in every country a freedom from malaria which is _purely
thermic_, for it is due simply and entirely to a sinking of the
temperature below the required minimum. Indeed, if the temperature in
winter rises above this minimum, there are often sudden outbreaks of
malaria. Sometimes, during very warm and dry summers, the heat extracts
all the humidity from the malarious soil, and thus procures for us a
freedom from the disease which is _purely hydraulic_.
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