Many of the infectious diseases--and nearly three fourths of all the diseases that children have are infectious--are caught, as we have seen, from germs that are carried in the air. That is one reason why so many infectious diseases are likely to begin with running at the nose, or sneezing, or cold in the head, or sore throat. The germs, having been breathed in with the air, catch on the sides of the nostrils or at the back of the throat, and start inflammation and soreness wherever they land. This is just the way that _measles_, _scarlet fever_, _chicken pox_, _whooping cough_, and _diphtheria_ begin. Nearly all colds in the head, and sore throats with coughing, are infectious; so the best thing to do whenever you have a bad cold in the head, or a sore throat, is to keep out in the open air as much as you can, until it is better. Of course, a cold is not such a serious thing in itself; but, if it is neglected, it may lead to some very dangerous troubles, particularly to inflammation of the lungs, and sometimes even of the kidneys or the liver or the heart. Several of these infectious diseases--measles, chicken pox, and scarlet fever, for instance--have a rash, or breaking-out, called an _eruption_, upon the skin. This is another thing easy to look out for; and if you see anyone with a rash upon his face and hands, it is a good thing to keep away from him and not let him touch you. Even if he should not have measles or scarlet fever or chicken pox, but only a disease of the skin itself, he still might spread the infection of that; for most diseases that cause a breaking-out upon the surface of the skin are infectious. Some of these infectious diseases are so common among children that they are called _Children's Diseases_, or the _Diseases of Infancy_, just as if it were natural for you to have them while you are children, and as if they were something that you have to have as a matter of course, before you grow up. But it isn't necessary at all to have them, if you will take care of yourselves and help your doctors and the Board of Health of your county or town or city to prevent their spreading. These diseases, although usually very mild, never do anyone any good whatever, and may do serious harm; for their poisons may stay in the blood and injure the heart or the kidneys or the nerves.