The virus of human tuberculosis, the tubercle bacillus of Koch, possesses narrowly limited morphological characteristics. It is readily recognized since Koch published his very accurate description of it in 1882, and especially since Ehrlich, shortly afterward, published a specific staining procedure. The tubercle bacilli are familiar to us as rods of varying length, but of fairly constant thickness, which occasionally show granular degeneration. I believe it is now everywhere accepted that the presence of these bacilli in a lesion in the human body indicates the tubercular character of that lesion. Leprous lesions with somewhat similar bacilli must, of course, be taken into account, but these, as a rule, can already be differentiated macroscopically. Recognizing this significance of the presence of Koch’s bacilli and applying all other known methods for the identification of tuberculosis, Dr. Naegeli of Zürich, working under the direction of Prof. Ribbert, was unable to discover at autopsy a single body over thirty years old in which there were not some signs of the occurrence of a tubercular infection. Between the ages of 18 and 30 there were 96%; between 14 and 18, 50%; between 5 and 14, 33%; and between 1 and 5 years, 17%, which showed the presence of tubercular lesions. In the bodies of infants under one year, on the other hand, definite tubercular signs were invariably absent. The astonishing results of these careful anatomical investigations have been verified by reports from pathological anatomists in other cities; yet at first sight, they seem to contradict all medical and other experience, for according to these results all of us assembled in this hall are tuberculous! But the perfected diagnosis of tuberculosis in the living person leaves no doubt that Naegeli’s figures, at least for thickly populated centers, possess general applicability. To be sure, if we count only the patients who come to the physician because of tubercular or supposedly tubercular symptoms, then our figures do not agree with Naegeli’s; nor will the statistics of living persons in whom we can discover tubercle bacilli agree with the autopsy statistics. The agreement, however, becomes very close if we make use of a diagnostic method furnished us by the second of Koch’s above-mentioned discoveries, the tuberculin injection.