There is definite relation between age and the infectious diseases.  In general, susceptibility is increased in the young; young animals can be successfully inoculated with diseases to which the adults of the species are immune, and certain human diseases, such as scarlet fever, measles and whooping cough, seem to be the prerogatives of the child.  It must be remembered, however, that one attack of these diseases confers a strong and lasting immunity and children represent a raw material unprotected by previous disease.  Where measles has been introduced into an island population for the first time, all ages seem equally susceptible.  All ages are equally susceptible to smallpox, and yet in the general prevalence of the disease in the prevaccination period it was almost confined to children, the adults being protected by a previous attack.  The habits and environment at different ages have an influence on the opportunities for infection.  There is comparatively little opportunity for infection during the first year, in which period the infant is nursed and has a narrow environment within which infection is easily controlled.  With increasing years the opportunities for infection increase.  When the child begins to move and crawl on hands and knees the hands become contaminated, and the habit of putting objects handled into the mouth makes infection by this route possible.  Food also becomes more varied, milk forms an important part of the diet, and we are now appreciating the possibilities of raw milk in conveying infection.  With the enlarging environment, with the school age bringing greater contact of the child with others, there come greater opportunities for infection which are partly offset by the increase in cleanliness.  The dangers of infection 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.