94 Foot-and-Mouth Disease. 



been successfully dealt with by isolation would appear to 

 warrant the conclusion that the disease is seldom air-borne for 

 long distances. 



The Cause of the Disease. — At the present day every 

 intelligent educated person is ready to admit that all con- 

 tagious and infectious diseases are caused by living things 

 which possess the power of multiplying within the bodies 

 of the men or animals attacked. No other assumption than 

 that the cause is a living thing — belonging either to the 

 animal or the vegetable kingdom — would afford a reasonable 

 explanation of the fact that the cause increases in amount, 

 often enormously, in the body of an individual suffering 

 from a contagious disease. 



In the case of many contagious diseases the actual thing 

 which is the cause of the mischief has been identified, and 

 the part which it plays proved beyond any doubt. That is true 

 of anthrax, glanders, and tuberculosis, to mention only a few 

 well-known diseases. In anthrax, for example, the cause is a 

 vegetable germ or bacterium which a good modern microscope 

 can make distinctly visible to the human eye in the blood or 

 other constituents of the body of a diseased animal. That 

 these so-called anthrax l)acilli are the actual cause of the 

 disease can be proved by growing successive crops of them in 

 test-tubes, and showing that when a few of these artificially 

 cultivated germs are inoculated or otherwise introduced into 

 the body of a susceptible animal, such as an ox or sheep, it 

 becomes infected with anthrax, and that the blood of the 

 animal thus experimentally infected when submitted to 

 microscopic examination contains inconceivable numbers of 

 bacilli identical with the few that were intentionally introduced 

 into its body. 



The causal connection between the bacillus of anthrax and 

 the disease of the same name is easily proved, because this 

 particular organism is of comparatively large size, easily 

 cultivated outside the body, and generally A^ery abundant in 

 the blood of infected animals at the time of death. 



In many other diseases of which the germ or bacterium is 

 now definitely known much greater difficulty was encountered 

 in providing the evidence necessary to prove the connection, 

 such difficulty generally depending on the verj* minute size of 

 the organism, or on the difficulty of growing it outside the 

 body. Pleuro-pneumonia of cattle is an example. In this case 

 the evidence necessary to secure a verdict against a particular 

 microbe has been obtained, and it is now generally accepted 

 that the disease is caused by an organism which the most 

 powerful modern microscope can only make visil)le to the 

 human eye as a mere point. This case is instructive, (1) 



