338 POPULAR SCIENCE MONTHLY. 



material. It was found by others, however, that the same animals 

 became tubercular if inoculated with apparently innocent mate- 

 rial, or simply if an open sore were kept on them. The matter 

 was thus still unsettled. The pathologists now came to the sup- 

 port '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 organ- 

 isms 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 cul- 

 tures introduced into healthy animals caused them to become dis- 

 eased, 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 contagious- 

 ness of the fevers, of cholera, and syphilis was established beyond 

 question before disease germs were discovered, sufiiciently con- 

 vincing evidence was not also at hand in the case of such a prev- 

 alent disease as consumption. The popular conception of c n- 

 tagion 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 iu speedy death or 

 recovery. A large proportion of those exposed are attacked, and 

 the exposure may have been very brief. The case is entirely dif- 

 ferent 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 indi- 

 vidual 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 



