Whether the entrance of tubercle bacilli into the tissue juices of an individual shall end disastrously or not, depends on a large number of factors. _Congenital_ differences in the degree of susceptibility undoubtedly play a subordinate rôle. A more important factor is the degree of virulence of the inoculated virus, and this may fluctuate within wide limits. Of great importance also is the amount of the virus and whether the inoculation is single, repeated, or almost continuous for a longer time. Of transcendent importance, however, are the age of the individual, his physiological or pathological condition at the time of infection, other previous diseases, mode of feeding, occupation, hygienic conditions of his surroundings, etc. In short, all that we understand by the term _acquired susceptibility_, or _acquired predisposition_. Finally, I must mention the location and character of the point of entrance of the virus. We see at once, therefore, that, in the analysis of a tubercular infection, it is not sufficient to have determined the presence of the three necessary factors, susceptibility, the tubercular virus, and the opportunity for infection. If we wish to understand the manifold variety of the reaction phenomena exhibited by an individual infected with tubercular virus, we must thoroughly study all the conditions above mentioned as well as a great variety of accidental conditions. Nowadays the term tuberculosis is applied to everything which constitutes a change in the organism excited by the entrance into it of tubercle bacilli. In statistics, galloping consumption is thus classed with tubercular local infection of a joint, gland, abdominal organ, of a little nodule in the skin of the face, etc. Calcified as well as other remains of previous lesions are classed as tubercular. We have every reason to praise as a great scientific advance the discovery that in manifestations so varied in their importance to life and health the etiological factor is the same. One must not, however, make the mistake of classing as one and the same thing tubercular infection and consumption and prognosticate an inevitable and painful end on making the diagnosis of tubercular infection. How little the facts justify such a course is seen in the statistics of bovine tuberculosis and will be seen also when I come to speak of the statistics of human tuberculosis.