material. It was found by others, however, that the same animals became tubercular if inoculated with apparently innocent material, or simply if an open sore were kept on them. The matter was thus still unsettled. The pathologists now came to the support of the contagionists by the discovery that the development and extension of the disease in contiguous cells and tissues and along the small vessels could only be explained by the presence of a contagious element. When, therefore, Koch demonstrated that the tubercles themselves were but the work of living organisms which had been introduced from without, a flood of light was thrown on the subject which cleared up all disputed points. He showed that not only was the diseased tissue crowded with these organisms, but that they were being constantly discharged in the sputum; that they could be cultivated in colonies free from all other germs outside the body; and that the pure cultures introduced into healthy animals caused them to become diseased, the bacilli being again found in their dead bodies. It may then be accepted as a settled fact that consumption is invariably produced by the introduction into the lungs from without the body of vegetable organisms, which, finding lodgment, multiply and eventually cause the destructive changes by which the death of the individual is accomplished. When it is remembered that consumptives have been expectorating countless millions of these organisms for ages, and that the cattle whose milk and flesh are used for food are not exempt from tuberculosis, no mystery need surround the source of any particular case. The mystery is rather that any of us escape. Nor could we, were it not that certain conditions, happily not always fulfilled, are necessary before the germs gain a foothold in the body.
It may strike some as extraordinary that while the contagiousness of the fevers, of cholera, and syphilis was established beyond question before disease germs were discovered, sufficiently convincing evidence was not also at hand in the case of such a prevalent disease as consumption. The popular conception of contagion and contagious disease has been derived from the behavior of such a disease as smallpox. It prevails often as an epidemic. Its onset is sudden and accompanied by pronounced symptoms. It runs a short and sharp course, and results in speedy death or recovery. A large proportion of those exposed are attacked, and the exposure may have been very brief. The case is entirely different with tuberculosis. Epidemics are almost unknown. The beginning of the disease is usually obscure and insidious; the course may extend over months and even many years, the individual being perhaps not seemingly very ill during much of the time. The majority of those exposed seem to have escaped, and of those attacked comparatively few are known to have been in
