f malaria
is caused by a parasite requiring forty-eight hours for its
development. The malarial attacks caused by this parasite then occur
every other day, when the parasite undergoes reproduction by division.
However, an attack may occur every day when there are two separate
groups of these parasites in the blood, the time of birth of one set
of parasites, with an accompanying malarial attack, happening one day;
that of the other group coming on the next, so that between the two
there is a daily birth of parasites and a daily attack of malaria. In
cases of malaria caused by one group of parasites the attacks appear
at about the same time of day, but when the attacks are caused by
different groups of parasites the times of attack may vary on
different days. In the worst types of malaria the parasites do not all
go through the same stages of development at the same time, as is
commonly the case in the milder forms prevalent in temperate regions,
so that the fever--corresponding to the stage of reproduction of the
parasites--occurs at irregular intervals.

In a not uncommon type of malaria the attacks occur every third day,
with two days of intermission or freedom from fever. Different groups
of parasites causing this form of malaria, and having different times
of reproduction, may inhabit the same patient and give rise to
variation in the times of attack. Thus, an attack may occur on two
successive days with a day of intermission.

The reproduction of the parasite in the human blood is not a sexual
reproduction; that takes place in the body of the mosquito.

When a healthy mosquito bites a malarial patient, the parasite enters
the body of the mosquito with the blood of the patient bitten. It
enters its stomach, where certain differing forms of the parasite,
taking the part of male and female individuals, unite and form a new
parasite, which, entering the stomach wall of the mosquito, gives
birth in the course of a week to innumerable small bodies as their
progeny. These find their way into the salivary glands which secrete
the poison of the mosquito bite, and escape, when the mosquito bites a
human being, into the blood of the latter and give him malaria.

=Distribution.=--Malaria is very widely distributed, and is much more
severe in tropical countries and the warmer parts of temperate
regions. In the United States malaria is prevalent in some parts of
New England, as in the Connecticut Valley, and in the course of