We find an explanation for this fact in the comparatively recent recognition of three substances, as yet not clearly understood, which are contained in a certain few articles of food, each one of which is essential to growth and normal health and well-being, though not necessarily concerned in the production of heat or energy. Various terms have been applied to these mysterious, but necessary substances, such as vitamines, accessory food substances as applied to all, or fat-soluble A, water-soluble B and water-soluble C to designate them separately. A surprisingly small amount of each of these substances is sufficient to meet the needs of an individual, but no one of these, even in this small amount, can be safely dispensed with, for if the diet is deficient, or lacking in one or more of them some form of nutritional disturbance will result. It may be severe enough to be diagnosed as a disease, or it may be only enough to keep the individual below a normal state of health. When the disturbance is profound enough to produce a definite, recognizable condition, it is designated as a deficiency disease, of which there are three: scurvy, beri-beri and xerophthalmia. With these are sometimes included rickets and pellagra. The exact cause of the two latter disorders is not definitely known but both are associated with faulty nutrition. Poor hygienic conditions may enter into the causation of rickets, and infection may be a factor in the occurrence of pellagra, but neither disease appears among those who are suitably fed while both diseases may be produced by faulty diet and both may be cured with suitable food. But probably of graver importance to the public welfare than the well defined nutritional disturbances, themselves, is the fact that between a state of good health and the level upon which a disease is recognizable is a long scale, along which are ranged an uncounted army of under-par, half-sick people. These are the ones who are tired, nervous, susceptible to infections, with feeble recuperative powers, and in general are more or less ineffective in the business of life. It is this borderline state, or as Dr. Goldberger terms it, “the twilight zone,” which cannot quite be called disease but is not health, that is serious to the masses, for diagnosed disease is given treatment, but nervousness, lack of energy and endurance, weakness and inefficiency are not treated; they are merely tolerated, as a rule. The sufferers fail to reach their highest possible development and they fail to be of highest value to society.