5. You may take germs into your body by putting pencils and other things into your mouth, and by drinking from a public drinking cup. 6. Spitting on the floor or pavement may scatter disease germs. 7. House flies and mosquitoes often spread diseases. CHAPTER XVII PREVENTING SICKNESS =212. How our body kills disease germs.=--We take disease germs into the body in three ways: first, through the mouth, second, through the nose, and third, through the skin. So we should watch the purity of our food, drink, and air, and should be careful about putting things into the mouth, and about the cleanliness of the skin. We often take a few disease germs into the body without catching a disease. This is because the white cells of our blood fight the germs and kill them (p. 53). If the body is hurt or weakened, the white blood cells may also be weakened so that they cannot kill the germs. We should take good care of the body so that every part of it may do its work well. We need not be able to run fast, or to lift heavy weights, but the best sign that every part of the body is in good order is to feel bright and wide-awake. Then our white blood cells will also be in good order and able to fight disease germs. =213. Catching cold.=--When we catch a disease, we often say that we have caught cold. We used to think that cold air and dampness were almost the only causes of taking cold, and this is the reason why we called many kinds of sickness by the name of colds. Now we know that we catch cold by taking disease germs into the body. The germs will not be able to grow unless the body is weakened in some way, as by cold and dampness. Yet if we are wet and cold, we shall not catch cold unless we take disease germs into the body. We do not get the germs from the outdoor air, for very few germs are there. We get them from the foul air of our houses when we come in to warm and dry ourselves. If the air of our houses were always as clean and pure as the outdoor air, we should hardly ever have colds. We can safely let the cold air blow on us if we are out of doors, but if we sit in a house, a small draft sometimes seems to make us take cold. This is because there are likely to be many disease germs in the house and few out of doors. Other things besides cold air and dampness may weaken the body, and so help us to take cold. If germs of colds are in a warm room, we may sit there and take cold even if we are not wet or chilled at all. The body may be weakened by poor food, wrong eating, or overwork, so that disease germs will easily grow in it. We take as many colds from these causes as from cold air and dampness.