tion
in the school period are now greatly lessened by medical inspection
and care of the school children. In the small epidemic of smallpox
which prevailed in Boston from 1881 to 1883, there was a sharp decline
in the incidence of the disease in children as soon as the school age
was reached, this being due to the demand of vaccination as a
condition for entrance into the schools. Many of the infectious
diseases are much milder in children than in adults. This is the case
in typhoid fever, malaria and yellow fever. The comparative immunity
of the natives to yellow fever in regions where this prevails seems to
be due to their having acquired the disease in infancy in so mild a
form that it was not recognized as such.

The infectious diseases are preeminently the diseases of the first
third of life. After the age of forty man represents a select
material. He has acquired immunity to many infections by having
experienced them. Habits of life have become fixed and there is a
general adjustment to environment. The only infectious disease which
shows no abatement in its incidence is pneumonia, and the mortality in
this increases with age. Between thirty-five and fifty-five man stands
on a tolerably firm foundation regarding health; after this the age
atrophies begin, the effects of previous damage begin to be apparent,
and the tumor incidence increases.

FOOTNOTE:
[1] This was the case of a woman, by occupation a cook, whose numerous
exchanges of service were accompanied by the appearance of cases of
typhoid fever in the families. This became so marked that an
examination was made and she was found to be a typhoid carrier and as
such constantly discharging typhoid bacilli. She is now isolated.

CHAPTER X

INHERITANCE AS A FACTOR IN DISEASE.--THE PROCESS OF CELL
MULTIPLICATION.--THE SEXUAL CELLS DIFFER FROM THE OTHER CELLS OF THE
BODY.--INFECTION OF THE OVUM.--INTRA-UTERINE INFECTION.--THE PLACENTA
AS A BARRIER TO INFECTION.--VARIATIONS AND MUTATIONS.--THE INHERITANCE
OF SUSCEPTIBILITY TO DISEASE.--THE INFLUENCE OF ALCOHOLISM IN THE
PARENTS ON THE DESCENDANTS.--THE HEREDITY OF NERVOUS
DISEASES.--TRANSMISSION OF DISEASE BY THE FEMALE ONLY.--HEMOPHILIA.--
THE INHERITANCE OF MALFORMATIONS.--THE CAUSES OF
MALFORMATIONS.--MATERNAL IMPRESSIONS HAVE NO INFLUENCE.--EUGENICS.

The question of inheritance of disease is closely associated with the
study of infection, and the general subject of heredity in its bearing
on d