Fatigue, then, ought to be one of our commonest medical-social diagnoses, and to help people out of it, one of the attempts that we most often make. In Dec., 1917, a dozen or more Y.M.C.A. boys consulted me in France, all with coughs, all wanting medicine to stop the cough, and most of them a good deal disappointed because they were told to go home and go to bed, told that they were tired, and that this fact depressed their resistance against bacteria, so that bronchitis or broncho-pneumonia resulted. The second point, then, that one wants to make about _fatigue is_, that it is _the commonest cause of infectious disease_. Pasteur's great discovery, which set modern medicine upon the right bases, sometimes gets twisted out of perspective. Sometimes we fail to realize that the seed may fall upon stony ground. The seed, of course, is bacteria, and its discovery was Pasteur's immense service to humanity. But Pasteur was so busy that he did not emphasize the truth that a seed can fall upon good ground or upon bad ground. When bacteria fall upon bad ground, that is, upon healthy tissue, they do not grow, they do not spring up and multiply. Tired tissues, as has been abundantly proved by animal experimentation, are prone to infection. They are good soil for the growth of bacteria. It is true generally; it is true locally. A part that has been injured, for instance, a part that has been bruised without any break in the skin, without the entrance of any infection from the outside, is damaged by something that hurts its resisting power as fatigue does. Such a part will often become inflamed, will often become subject to the action of bacteria which must have been in the body already, but which had been kept on the frontier by our powers of resistance. Our "powers of resistance," then, which we cannot more definitely name, which we do not as yet know to be identified with leucocytes or with anything else, can get tired. When they get tired we "catch" a cold or a diarrhea, or a hundred things which seem to have nothing to do with fatigue, but _have_ nevertheless.