It is for this reason that the social worker is above all others the person who can convey life-saving information to the public in an effective way. A considerable amount of this precious knowledge is now possessed by the medical profession; but it is shut away useless, unavailable, in medical libraries and in doctors' minds. The social worker can fight disease by spreading the contagion of medical truth. She can multiply the foci from which truth can spread still more after she is gone, just as disease is redistributed again and again from new nests of infection. The _prognosis of disease_, like its causation, is a subject on which the social worker should know almost as much as the doctor. This is possible because medical knowledge on this subject is still so very limited. For the purposes of one who has to combat the poverty, sorrow, idleness, and corroding fears which disease produces, knowledge of prognosis is a most useful tool. For example: if one is to make plans for the care of a group of children during their mother's illness, one must have some idea how long that illness is going to last. If it affects the bread-winner of the family, how long will he or she be disabled, and how completely; what are the hopes of ultimate and complete recovery; will chronic invalidism follow; is it worth while in this particular disease to spend a great deal of money and time in trying to achieve a complete cure, or is cure so improbable and at best so incomplete that our resources can be expended more wisely in other directions? A knowledge of prognosis will help the home visitor greatly in the solution of such problems. But it must be added that such knowledge as she already possesses about the prognosis of a disease, such as tuberculosis or heart trouble or kidney trouble, must always be supplemented by all the information that she can gain from the doctor as to the present prognosis in the case of the particular patient with whom the social worker has to deal. For the general prognosis of a disease is greatly modified by the particular circumstances in each individual case. Physicians are not at all eager to impart their knowledge about prognosis, because this knowledge is so limited and so faulty. No scientific man likes to make definite statements upon so indefinite and hazy a matter as prognosis. Nevertheless, it is essential for the patient's good that the doctor should be asked to give her as clear and definite a statement as is possible for him to make with the facts that he possesses. For it is only upon the basis of such a statement that an intelligent plan of social treatment can be constructed.