The human body is a complex and intricate organism, in some wonderful and entirely incomprehensible way integrating the activities of all these billions of other living organisms. Each and every one of these latter has its function to fulfill, and the life of the individual body is a life of health so long as the unity of all its organisms is maintained. Outside of the body are millions of hostile organisms assaulting it continuously; and the problem of health is the problem of enabling it to make headway against its enemies for as long a period as possible. Every act of a human being has its effect upon this battle; at every moment of your life you are either strengthening the power of your own organism or strengthening your enemies. Once the organism is unable to beat back its enemies, health begins to fail and death and complete disintegration is the ultimate result. It must be understood that the peril of these hostile germs is not merely that they devour the substance upon which the body’s own organisms have to be nourished. If that were all, they might remain in the body as parasites, and by taking additional nourishment a man might sustain life in spite of them. Nor is it even that they multiply with such enormous rapidity; the peril is that they throw off as the products of their own activity a number of poisons, which are as deadly to the human body as any known. These poisons are produced much more rapidly than they can be eliminated from the system, and so they fill the blood, and death ensues. Thus the problem becomes clear. In the first place, what can we do to keep disease germs from securing entrance to the body; and second, what can we do to strengthen the body’s army of defense so that the fate of any which do find entrance may be immediate destruction? HEALTH, LIKE DISEASE, IS CATCHING In actual practice it is found that the second problem is by far the more important one. Some germs we can avoid. If we boil all the water that we drink we will not be very apt to have typhoid. If we exterminate rats and mosquitoes and flies and fleas, we will not have yellow fever, or malaria, or plague. But we cannot hope to do this at present in the case of such diseases as, for instance, consumption, grippe, and influenza. If we live in a city, we take into our lungs and throat millions of the germs of these diseases every day. Therefore the one hope that is left is to keep ourselves in such a condition of health that the army of our bodies shall be able to destroy these germs. When the blood is in a healthy condition, the white cells are numerous, powerful, and active, but when the blood flows stagnantly, or when it is impoverished, then the white cells are few and the forces of disease obtain a foothold.