But it will perhaps be said, "If the tubercle bacilli are so widely diffused, why do we not all acquire tuberculosis, and why was the world not long since depopulated?" In order to explain this matter I must ask the reader to look with me for a moment at some of the body's natural safeguards against bacterial and other invaders from the air. It has been found that a person breathing in germ and dust-laden air through the nose breathes out again air which is both dust and germ free. The air passages of the nose are tortuous, and lined with a moist membrane, against which the air impinges in its passage. On these moist surfaces most of the solid suspended particles, the germs among them, are caught and held fast, and may be thrown off again by secretion. In breathing through the mouth this safeguard is not utilized. Again, the upper passages leading to the lungs are lined with a delicate membrane of cells, whose free surfaces are thickly beset with tiny hairlike projections. These projections are constantly moving back and forth with a quick sweep, in such a way that they carry small particles which may have escaped the barriers above up into the mouth, from which they may be readily discharged. In this way much of the evil of breathing dust and germ-laden air is averted. But in spite of these natural safeguards a great deal of foreign material does, under the ordinary conditions of life indoors or in dusty places, find lodgment in the delicate recesses of the lungs. The body tolerates a good deal of the deleterious material, but its overtasked toleration fails at last, when serious disease may ensue.
When ordinary forms of living bacteria get into the tissues of the body, a very complex cellular mechanism, not fully understood, usually results in their destruction and ultimate removal. In the presence of the tubercle bacillus the body cells are often able to build a dense enclosing wall around the affected region, shutting it off from the rest of the body. This is one of the modes of natural cure. The body cells are sometimes able, if sustained by nourishing food and an abundance of fresh air, to carry on, year after year, a successful struggle with the invading germs, so that the usefulness and enjoyment of life are but little interfered with. Finally, a certain proportion of human beings seem to be endowed at birth with some as yet unknown quality in the cells or fluids of the body which naturally unfits them for the life uses of the tubercle bacillus, and so renders the individual for longer or shorter periods practically immune.