e acquired a habitude to the disease which prevents the
normal effects.

Diphtheria is prevented in much the same way, except that in this case
horses are used, their blood being strengthened to resist the disease by
successive inoculations of the diphtheria poison. It is probable that
all the bacterial diseases which exert their influence through the
transmission of toxins in the blood may be counteracted by the
production of an antitoxin when once the method of building up this
antitoxin has been learned. At present, rabies, tetanus, diphtheria, and
cerebrospinal meningitis are the four diseases for which antitoxin is
made commercially and generally used. For a great many years, scientists
have labored without success to find an antitoxin for consumption, and
within the last year extensive experiments have been made in the
American army on the use of antitoxin for typhoid fever.

_Natural immunity._

It may be worth noting that not all resistance to specific diseases
needs to be acquired in the roundabout way just described. The state of
being free from disease is known as immunity, and the way of securing
immunity just described is known as artificial immunity. This artificial
immunity may also be obtained in the course of events by having the
disease as a child, thereby generating the antitoxin in one's own body
instead of in the body of some cow or horse or rabbit.

There is, however, a natural immunity which is due to long-continued
environment or to protracted heredity. The negroes in the South have,
by a lifelong proximity and struggle with the disease, acquired a
practical freedom from typhoid fever, although it remains with the negro
sufficiently to form a focus for the spread of the disease among others
not equally immune. Creoles in yellow-fever districts have a natural
immunity from the hookworm disease, although probably the class are
responsible for its generous transmission to the poor whites with whom
they associate. Racial immunity from certain diseases may be shown by
statistical studies.

_Chemical poisons._

Instead of the introduction of toxins into the body by the agency of
bacteria, it is quite possible for chemical poisons, not formed
originally by bacteria, to be set free in the body. Sulphate of copper,
for instance, is essentially a mineral poison which acts on the human
system in such a way as to produce death, and certain other mineral
substances may be mentioned, such as phosphorus,