Commoner and not less important than the contagious diseases that I have just mentioned are industrial diseases, or diseases aggravated by the conditions of industry. A physician may serve for many months in a dispensary without seeing a case of smallpox, of trichiniasis, or of typhoid fever, or feeling it his duty to set in motion the forces that I have just mentioned for rooting out the sources of contagion and preventing their further spread. But he cannot serve a month in any well-attended dispensary without seeing cases of industrial disease in the narrow sense, such as lead poisoning, or of independent disease aggravated by the conditions of industry, such as the functional neuroses of cigar-makers or of telephone operators. With such diseases, as with the infectious and contagious diseases, the presence of one case in the clinic should lead straight to the inference that there are others elsewhere, out of sight but no less important from the point of view of public good. This conclusion should lead in turn to the search through a social worker for the cases of disease which do not present themselves to any physician, which may be totally unknown even to the patient himself, yet which are important to the health of the nation. Difficult though this field of industrial disease has shown itself to be, difficult though it is to separate out that portion of the patients' complaints which can justly be referred to the conditions of his work, and to distinguish it from the portions which are due to the way he lives, to his inheritance, to his habits or to diseases like tuberculosis and syphilis which may have been acquired without any connection with his work,--nevertheless we must try to disentangle and to recognize the elements in this knotty problem. And we can hardly fail to see that the social worker is an essential and logical assistant in the processes of investigation which we must carry out. If we can ever unravel the tangled skein of causes and effects whereby the hours of work, the strain of work, the patients' heredity and his home conditions, all combine to produce the symptoms of disease, it will be through such intimate, prolonged, detailed studies as the social worker can carry out, especially if she becomes a friend of the family. The doctor in his hours of consultation at the dispensary certainly can never do it. The official agent of the Board of Health, perhaps feared, certainly not a natural confidant for the family, may easily miss the truth which the social worker unearths, provided always she succeeds in differentiating herself altogether from the impersonal and professional investigator, and gradually becomes in the mind of the family and in truth their friend.