The social worker, then, should share such knowledge as the medical profession possesses about the causation of infectious disease, about direct personal contagion, and also about the indirect _methods by which disease is conveyed_ from person to person through insects or through instruments and utensils, such as the barber's razor, the family towel, or the public drinking-cup. She should be familiar with the small body of knowledge which we possess upon the transmission of disease by drinking-water, by milk, and other kinds of food. She should appreciate our still smaller body of knowledge about the relation of disease to climate, to weather, and to other physical agents such as the extreme heat and cold produced by some industrial processes, and the action of X-rays. In addition to this definite and specific knowledge of causes, she should know the generally accepted views of the medical profession on the subject of _bodily resistance_, _immunity_, inheritance, the diseases and perversions of metabolism, and the other non-bacterial factors in the production of disease. Above all, she should realize the _multiplicity of causes_ which science more and more clearly recognizes in their single result. She should learn both by precept and by experience that for a single fact such as disease or health there are always many causes, so that any one who points confidently to a single cause, such as cold, fatigue, bacteria, or worry as a sufficient explanation of a person's disease, is almost certain to be wrong. Obviously, this truth bears a close relation to what is to be said on the "historic and catastrophic points of view." Chapter III. The importance of teaching the social worker all that is known about the transmission and causation of disease is due to the following fact: whatever we succeed in accomplishing in our efforts at _preventive medicine_, whatever we do to nip disease in the bud or to check the spread of epidemics, is due to our knowledge of the causes of disease. The instructions of the doctor at the dispensary can accomplish but little in this field when compared with the detailed teaching of the social worker in the patient's house, in his workshop, in the schools and factories where disease is spread so much more frequently than in the dispensaries. If we hope to show people how they can avoid the disasters of illness, our teaching should be given in the very place where these disasters most often occur. There we can illustrate and demonstrate with the objects in sight what is to be done and to be avoided.