It is said by Hirsch that about one-seventh of the mortality that exists among civilized men is due to tuberculosis in its various forms; and when we consider the dreadful mortality due to this group of diseases, there is rather a remarkable fact that confronts us in studying the life history of the tubercle bacillus, namely, that it is apparently, when compared with many other species of bacilli and bacteria, very much less virulent and deadly.
Let us compare for a moment the tubercle bacillus with the Klebs-Loeffler bacillus of diphtheria. The tubercle bacillus, it is true, can be inoculated on raw and cut surfaces, but only with great difficulty and under exceptionally favorable conditions which aid in its development. Even when it does develop under favorable conditions of soil and habitat, its powers of growth and multiplication are slow and feeble, when compared with many other pathogenic micro-organisms. Compare