s in which it is brought about are still in many cases far from
clear.

Immunity may be _natural_ or _acquired_. By natural immunity is meant
an inherited characteristic by which all individuals of a species are
immune to a certain disease. The natural immunity of certain species of
animals to the diseases of other animals is well known. Man is immune to
many diseases of lower animals, and they in turn are immune to many
diseases of man. Cattle, for instance, are immune to typhoid and yellow
fever, while man shows high resistance to rinderpest and Texas fever;
both, however, are susceptible to tuberculosis, to which goats are
immune. There are all gradations of immunity within the same species.
Moreover, certain individuals have a personal immunity against diseases
to which others of the same race or species are susceptible.

Immunity may be _acquired_ in several ways. It is commonly known that
one attack of certain communicable diseases renders the individual
immune for a varying length of time, and sometimes for life. Among these
diseases are smallpox, measles, whooping-cough, scarlet fever, infantile
paralysis, typhoid fever, chicken-pox, and mumps; erysipelas and
pneumonia on the other hand appear to diminish resistance and to leave a
person more susceptible to later attacks.

Again, in some cases immunity may be artificially acquired by
introducing certain substances into the body to increase its
resistance. Examples of this method include the use of antitoxin as a
protection against diphtheria, of sera in pneumonia and other
infections, and vaccination against smallpox and typhoid fever whereby a
slight form of the disease is artificially induced. Laboratory research
goes on constantly, and doubtless many more substances will eventually
be discovered that will reduce human misery as vaccines and antitoxin
have already reduced it.

Vaccination and inoculation have saved thousands of lives. Smallpox,
once more prevalent than measles, was the scourge of Europe until
vaccination was introduced. During the 18th century it was estimated
that 60,000,000 people died of it, and at the beginning of the 19th
century one-fifth of all children born died of smallpox before they were
10 years old. In countries where vaccination is not practised the
disease is as serious as ever; in Russia during the five years from
1893-97, 275,502 persons died of smallpox, while in Germany where
vaccination is compulsory, only 8 people died