lk, water, food, and
insects may be controlled by public action, that is, by specific
measures taken by a large group of people in order to protect the
individual. Such action constitutes _public sanitation_. There is,
however, a large group of diseases, chiefly sputum-borne, that cannot be
controlled except by individual action. Such individual action
constitutes a large part of _personal hygiene_.

The whole problem of controlling infections sounds simple, depending as
it does for the most part upon unpolluted water, milk, and food,
extermination of certain insects, and cleanliness in personal behaviour.
In practice the problem is not so easy. Public sanitation has performed
miracles in the past, and will do much in the future; behaviour,
however, will continue to be influenced by many factors, social and
economic as well as personal. Ignorance of the laws of health is an
obstacle to progress, but in modern conditions even the instructed may
be unable to control their ways of living and working. Indeed, such
control is at present limited to the privileged few. On the ignorant and
the poor, those least able to bear it, society loads the heaviest burden
of sickness. Only when ignorance and poverty are abolished, as one day
they will be, can the final stage be reached in the fight for public
health.

THE NON-COMMUNICABLE DISEASES

In this group is included a great variety of maladies. Of some the
causes are known, while in the case of others, origin, prevention, and
remedy are still obscure. Here belong defects in structure of the body,
both hereditary and acquired; insanity and other nervous diseases; new
growths, like tumors and cancer; disturbances of bodily processes, as
malnutrition and gout; and the important class of degenerative diseases,
like arteriosclerosis, in which tissues become hardened and fibrous and
hence less able to perform their normal functions.

The degenerative diseases are playing a menacing part in national
health. The average length of life in the United States has shown a
marked increase it is true, during the last 40 years. But this gain
represents chiefly the saving of life through prevention of communicable
diseases, especially among babies and children; among people who have
passed the 30th year on the other hand, death rates are actually
increasing. This increase is most marked after the age of 45, and is
caused chiefly by the increase of cancer, and of degenerative diseases
of the