til it is free from
the gas. This will require a day or more of thorough ventilation.

*Facts Relating to the Spread of Certain Diseases.*--The problem of
preventing disease in general often resolves itself into the problem of
preventing the spread of some particular disease. It is then of vital
importance to know the special method by which the germs of this disease
leave the body of the patient and are conveyed to the bodies of others.
Some of these methods are novel in the extreme, and are not at all in
accord with prevailing notions. Particularly is this true of that disease
known as

*Malaria, or Malarial Fever.*--This disease, so common in warm climates and
also prevalent to a large extent in the temperate zones, is due to animal
germs (protozoa), which attack and destroy the red corpuscles of the
blood. These germs, it is found, pass from malarial patients to others
through the agency of a variety of mosquitoes known as _Anopheles_. In
sucking the blood of a malarial patient, the mosquito first infects her
own body.(131) In the body of the mosquito the germs undergo an essential
stage of their development, after which they are injected beneath the skin
of whomsoever the mosquito feeds upon. For the spreading of malaria, then,
two conditions are necessary: first, there must be people who have the
disease; and second, there must be in the neighborhood the special variety
of mosquito that spreads the disease. If either condition be lacking, the
disease is not spread. The malarial mosquito (_Anopheles_) may be
distinguished from the harmless variety (_Culex_) by the position which it
assumes in resting, as shown in Fig. 170.

[Fig. 170]

Fig. 170--*Mosquitoes* in resting position. (From Howard's _Mosquitoes_.)
On left the malarial mosquito (_Anopheles_); on the right the harmless
mosquito (_Culex_).

*Remedies against Mosquitoes.*--The natural method of preventing the spread
of malaria is, of course, the destruction of mosquitoes. This is
accomplished by draining pools of water where they are likely to breed,
and by covering pools of water that cannot be drained with crude petroleum
or kerosene. The kerosene, by destroying the larvae, prevents the
development of the young. In communities where such measures have been
diligently carried out, the mosquito pest has been practically eliminated.
Other methods are also under investigation, such as the st