We need not, however, idly fold our hands and become fatalists who see the inevitable destruction of the human race by tuberculosis. _Tubercular infection does not by any means signify tubercular consumption._ It is just this enormous diffusion of tubercular infection which demonstrates, better than anything else, the curability of many tubercular diseases, their _liability to spontaneous cure_; for I must admit that I have little faith in the curative action of any of the numerous methods of treating tuberculosis. Here also the maxim formerly applied to diphtheria holds true, “mild cases go on to recovery, severe infections are fatal”; and here as in diphtheria I was confronted by the question whether there are ways and means to prevent severe infections with bad prognoses, or to convert severe infections into mild ones with favorable prognoses. You will be able to answer this question yourself if I now give you the results of my experimental investigations on the occurrence and prevention of tubercular consumption. I shall begin by discussing the requirements necessary for the development of tubercular consumption in man. But first of all I must say that according to my ideas there has not yet been a single well-authenticated case in which pulmonary consumption has originated in adult persons as the result of a tubercular infection developing epidemiologically, i.e., under essential conditions for infection occurring in nature. Even counting those cases in which mortuary assistants, butchers, and laboratory workers have been severely infected through the subcutaneous tissues, I still fail to find any proof that a traumatic infection has caused pulmonary consumption in an individual not yet infected with tubercle bacilli. I am well acquainted with the argument by which it is sought to prove that pulmonary consumption may develop as a result of inspiration of particles of dust or moisture containing tubercle bacilli. The facts on which this argument is based are the greater occurrence of tuberculosis and a higher mortality rate from that disease among nurses, occupants of houses in which there are pronounced cases of phthisis, among the inmates of prisons, etc. But, considering the figures previously given, showing the enormous diffusion of tuberculosis, the objection is surely justified that the persons thus dying of consumption already had a tubercular focus in the lungs and that this pulmonary disease, under a mode of life favorable to tuberculosis, was converted into florid phthisis.