By their effects upon the endocrines, public health influences like food, clothing, sleep and overpressure and last but not least, disease, the so-called diseases of childhood, possess a tremendous importance in limiting the output of the educable.  They act to subtract from and so to lower the rating, the capacity of the germ-plasm.  Most material and vital of these influences are the common diseases of children, for they strike directly at the glands of internal secretion.

Measles, scarlet fever, diphtheria, mumps, and the others have long been accepted as providential visitations for sins known or unknown.  That children had to have them and were better off when they had them has become part of the tradition of the laity, fostered by the lazy ignorance of previous medical generations.  But today we are beginning to ask ourselves why children must have these endemic infections of their age.  The pathologist goes farther and asks the reason for certain apparent immunities.  He asks why the little boy who sleeps with his brother sick with scarlet fever does not contract the disease, even though not protected by a previous attack.

Determining why susceptibility to a special disease in a particular case exists will constitute the greatest line of advance for the understanding and prevention of disease, and so the perfection of public health.  In the last influenza epidemic countless physicians were puzzled by the spectacle of men and women in the pink of condition carried off in twenty-four hours while puny associates were either passed over, or pooh-poohed their colds.  Pathologists have spent their energies fruitfully upon the infectious causes of disease, the microbes and parasites especially.  But now, having solved most of those problems, the vital question of why an organism permits itself to be attacked is pushing itself to the front.  Why a peculiar ailment selects its victim, why the bacillus finds a fertile soil, is the neglected problem, which must be solved before the abolition of disease and its carriers will be remotely conceivable.

Long ago, Hippocrates, revered founder of the art of medicine, recognized that there was a specific affinity of disease for individuals with more or less the same characteristic somatic and psychic traits and trends.  Tuberculosis, for instance, was noted for its frequency in long-skeletoned, thin persons, remarkably optimistic.  And the plethoric, choleric nature of the sufferer from gout has become proverbial.  Before the era of the great bacteriologic discoveries of the eighties and nineties, the concordance of esoteric racial and personal markings was a great help in diagnosis to the physician.  For he realized, though he sometimes credited it to his clinical intuition, that it was a certain type of personality that was liable to the specific disease.