This is known as _fission_ or "splitting," and is of interest because this is the way in which the little cells that make up our own bodies increase in number; as, for instance, when a muscle is growing and enlarging under exercise, or when more of the white blood cells are needed to fight some disease. Remember that we and the disease germs are both cells; and that, if they are numbered by millions, we are by billions; and that we are made up of far the older and the tougher cells of the two. Except in a few of the most virulent and deadly of fevers, like the famous "Black Death," or _bubonic plague_, and lock-jaw, or _tetanus_, ninety-five times out of a hundred when disease germs get into our bodies, it is our bodies that eat up the germs instead of the germs our bodies. Keep away from disease germs all that you reasonably and possibly can; but don't forget that the best protection against infectious diseases, in the long run, is a strong, vigorous, healthy body that can literally "eat them alive." Grow that kind of body, keep it perfectly clean inside and out, and you have little need to fear fevers, or indeed any other kind of disease; for you will live until you are old enough to die--and then you'll want to, just as you want to go to sleep when you are tired. Remember that this fight against the fevers is a winning fight, this study of disease germs a cheering and encouraging one, because it will end in our conquering them, not merely nine times out of ten, but ninety-nine times out of a hundred. We are not making this fight just to escape death; what we are fighting for is to live out a full, useful, and happy life. And we already have five chances to one of gaining this, and the chances are improving every year; for science has already raised the average length of life from barely twenty years to over forty. Broadly speaking, if you will keep away from every one whom you know to have an infectious disease; wash your hands always before you eat, or put anything into your mouth; keep your fingers, pencils, pennies, and pins out of your mouth,--where they _don't_ belong; live and play in the open air as much as possible and keep your windows well open day and night, you will avoid nine-tenths of the risks from germs and the dangers that they bring in their wake.