Not only should the teeth be kept thoroughly clean and sweet for their own sake, but also for the sake of the stomach and the health of the blood and the whole body. The mouth, being continually moist and warm and full of chinks and pockets, furnishes an ideal breeding ground for all kinds of germs; and the average, uncleansed human mouth will be found to contain regularly more than thirty different species of germs, each numbering its millions! Among them may sometimes be found the germs of serious diseases such as pneumonia, diphtheria, and blood-poisoning, just waiting, as it were, their opportunity to attack the body. In fact, a dirty, neglected mouth is one of the commonest causes of disease. CHAPTER XXVI INFECTIONS, AND HOW TO AVOID THEM What Causes Disease. The commonest and most dangerous accident that is likely to happen to you is to catch some disease. Fortunately, however, this is an accident that is as preventable as it is common. Indeed, if everybody would help the Board of Health in its fight against the spread of the common "catchable" diseases, these diseases could soon be wiped out of existence. Every one of them is due to dirt of some sort; and absolute cleanness would do away with them altogether. Diseases that are "catching," or will spread from one person to another, are called _infections_; and all of them, as might be supposed from their power of spreading, are due to tiny living particles, called _germs_--so tiny that they cannot be seen except under a powerful microscope. Nine-tenths of these disease germs are little plants of the same class as the moulds that grow upon cheese or stale bread, and are called bacteria, or bacilli. The different kinds of bacteria, or bacilli, are usually named after the diseases they produce, or else after the scientists who discovered them. For instance, the germ that causes typhoid fever is called the _bacillus typhosus_; that which causes tuberculosis is called the bacillus tuberculosis; while the germ of diphtheria known as the _Klebs-Loeffler bacillus_, after the two men who discovered it. A few kinds of disease germs belong to the animal kingdom, though all germs are so tiny that you would have to have a very powerful microscope to tell the difference between the animal germs and the bacilli, or little plants. Most of these animal germs are called _protozoa_ and cause diseases found in, or near, the tropics, like malaria and the terrible "sleeping sickness" of Africa. Smallpox, yellow fever, and hydrophobia--the disease that results from the bite of a mad dog--are also probably due to animal germs.