One thing I should like to urge you to do if you happen to get one of these "children's diseases"; and that is, to stay in bed or out of school or away from work just as long as your doctor tells you to. This is important, because it is very dangerous indeed to become over-tired or overheated or chilled, or to get your feet wet or romp too hard or sit up too late, before you have fully recovered; and you will not have fully recovered until at least three or four weeks after you are able to be out of bed. But if you take good care of yourselves for three or four weeks after measles or chicken pox or whooping cough or a very bad cold, you will avoid almost all danger of their poisons injuring your heart or kidneys or nerves, and causing chronic diseases, like Bright's disease or heart disease, later in life. Perhaps now I have told you enough about poisons and sickness. You must not be frightened about them. I have told you these things so that you may understand why you must bathe, and brush your teeth, and wash your face and hands, and wear clean clothes, and breathe fresh air, and keep your windows open, and play out of doors--in fact, keep your bodies clean inside and out. I know you will be glad enough to do these things, troublesome though some of them may be, if you know the reason why. The best of it is that when you keep perfectly clean and healthy, not even the "Great White Plague" and cold seeds, or germs, can hurt you, even though they get into your mouth or nose; for Mother Nature gives healthy bodies the power to kill germs, and quite without our knowing it. [Illustration: ENJOYING "ALL OUTDOORS" Very discouraging to disease germs!] III. PROTECTING OUR FRIENDS If you knew that some of your little friends were sick with an infectious disease like measles or scarlet fever, of course you would keep away from them, so as to avoid catching the disease. And if they knew that they had a disease that was infectious, of course they would want to let all their friends know of it, so as to prevent them from coming and catching it. But how can they let all their friends know? Sick people don't feel like writing letters; and, even if they did, some diseases can be carried in letters. So that might not be at all a friendly thing to do. This has always been the greatest difficulty in preventing the spread of infectious diseases--how to let other people know. So about fifty or sixty years ago, people got together and decided that the best thing to do was to appoint an officer known as a _Health Officer_, or a committee known as a _Board of Health_, in each town and in each county, whose business it should be to find out cases of infectious disease, and to warn other people against them.