*Causes of Disease.*—Disease, which is some _derangement of the vital functions_, may be due to a variety of causes. Some of these causes, such as hereditary defects, are remote and beyond the control of the individual. Others are the result of negligence in the observance of well-recognized hygienic laws. Others still are of the nature of influences, such as climate, the house in which one lives, or one’s method of gaining a livelihood, that produce changes in the body, imperceptible at the time, but, in the long run, laying the foundations of disease. And last, and most potent, are the minute living organisms, called microbes or germs, that find their way into the body. Although there are two general kinds of germs, known as _bacteria_ (one-celled plants) and _protozoa_ (one-celled animals), most of our germ diseases are caused by bacteria. *Effects of Germs.*—While there are many kinds of germs that have no ill effect upon the body and others that are thought to aid it in its work, there are many well-known varieties that produce effects decidedly harmful. They gain an entrance through the lungs, food canal, or skin, and, living upon the fluids and tissues, multiply with great rapidity until they permeate the entire body. Not only do they destroy the protoplasm, but they form waste products, called _toxins_, which act as poisons. Diseases caused by germs are known as infectious, or contagious, diseases.(129) The list is a long one and includes smallpox, measles, diphtheria, scarlet fever, typhoid fever, tuberculosis, la grippe, malaria, yellow fever, and others of common occurrence. In addition to the diseases that are well pronounced, it is probable that germs are responsible also for certain bodily ailments of a milder character.(130) *Avoidance of Germ Diseases.*—The problem of preventing diseases caused by germs is an exceedingly difficult one and no solution for all diseases has yet been found. One’s chances of avoiding such diseases, however, may be greatly enhanced: 1. By strengthening the body through hygienic living so that it offers greater resistance to the invasions of germs. 2. By living as far as possible under conditions that are unfavorable to germ life. 3. By understanding the agencies through which disease germs are spread from person to person. *Conditions Favorable and Unfavorable for Germs.*—Conditions favorable for germ life are supplied by animal and vegetable matter, moisture, and a moderate degree of warmth. Hence disease germs may be kept alive in damp cellars and places of filth. Even living rooms that are poorly lighted or ventilated may harbor them. Water may, if it contain a small per cent of organic matter, support such dangerous germs as those of typhoid fever. Fresh air, sunlight, dryness, cleanliness, and a high temperature, on the other hand, are destructive of germs. The germs in impure water, as already noted (page 165), are destroyed by boiling.