INFECTION IN DISEASE. 745 



The microbes will be thus seen to have selective power or 

 chemical affinities for particular parts of the body — the 

 scarlatina contagium for the tonsil, diphtheria for mucous 

 membrane of mouth, typhoid for bowel, glands, &c. 



The diseases of animals which are transmissible to man 

 do not appear to be communicable through the air, but only 

 by direct inoculation or direct contact. 



In whatever way the contagium is taken into the system, 

 it has the power of multiplying therein, and of producing 

 certain symptoms indicative of its onward progress. 



The first symptom of the invasion of the pathogenic 

 microbe is a rise of temperature of the body, which indicates 

 that increased chemical action or metabolism is going on in 

 the system in an unusual manner. Should the infection 

 progress other symptoms show themselves, which prove the 

 particular contagium which has given rise to the disease. 

 And as each contagium only produces its own particular set 

 of signs and symtoms — as from a seed only a particular plant 

 is produced — so we have the specific infectious diseases 

 defined accordingly. 



The diseases that we have mostly to deal with, and which are 

 officially regarded, according to the Public Health Act, to be 

 " dangerous, infectious, or contagious," are smallpox, Asiatic 

 cholera, plague, yellow fever, typhus fever, measles, scarlatina, 

 typhoid fever, diphtheria, and whooping-cough. 



As our knowledge increases, others no doubt will be added 

 to this list; and already in some countries tuberculosis is 

 regarded as belonging to the type of "highly infectious 

 diseases." 



Before coming to a decision that any particular bacillus 

 was the cause of disease, Koch was guided by three rules 

 which he laid down as his guides in his investigations, and 

 it would be well if these rules were steadily kept in view 

 with regard to all cases of infectious disease. The first principle 

 is, that the bacillus must be found invariably in each case 

 of the particular diseases ; secondly, that it is not found in 

 any other disease than the one to which it relates ; thirdly, 

 that the micro-organism can be isolated from the diseased 

 body and cultivated outside in nutrient media, when a pure 

 culture can be obtained from which the disease can be repro- 

 duced with certainty when inoculated into the animal system. 

 The circle of proof would be thus complete, and the con- 

 nection of the micro-organism with the particular disease 

 clearly established. This series of proofs has been demon- 



