while it is prevalent in the tropics and subtropics all the year
round, but more commonly in the spring and fall of these regions. The
older ideas, that malaria was caused by something arising in vapors
from wet grounds or water, or by contamination of the drinking water,
or by night air, or was due to sleeping outdoors or on the ground
floors of dwellings, are only true in so far as these favor the growth
of the peculiar kind of mosquitoes infected by the malarial parasites.
Two essentials are requisite for the existence of malaria in a region:
the presence of the particular mosquito, and the actual infection of
the mosquito with the malarial parasite. The kind of mosquito acting
as host to the malarial parasite is the genus _Anopheles_, of which
there are several species. The more common house mosquito of the
United States is the _Culex_. The _Anopheles_ can usually be
distinguished from the latter by its mottled wings, and, when on a
wall or ceiling, it sits with the body protruding at an angle of 45 deg.
from the surface, with its hind legs hanging down or drawn against the
wall. In the case of the _Culex_, the body is held parallel with the
wall, the wings are usually not mottled, and the hind legs are carried
up over the back.

When a mosquito infected with the malarial parasite bites man, the
parasite enters his blood along with the saliva that anoints the
lancet of the mosquito. The parasite is one of the simplest forms of
animal life, consisting of a microscopical mass of living, motile
matter which enters the red-blood cell of man, and there grows,
undergoes changes, and, after a variable time, multiplies by dividing
into a number of still smaller bodies which represent a new generation
of young parasites. This completes the whole period of their
existence. It is at that stage in the development of the parasite in
the human body when it multiplies by dividing that the chills and
fever in malaria appear. What causes the malarial attack at this point
is unknown, unless it be that the parasites give rise to a poison at
the time of their division. Between the attacks of chills and fever in
malaria there is usually an interval of freedom of a few hours, which
corresponds to the period intervening in the life of the parasite in
the human body, between the birth of the young parasites and their
growth and final division, in turn, into new individuals. This
interval varies with the kind of parasite. The common form o