CHAPTER IV. - 



Methods of Transmission; Special Qualities of the Pathogenic Bacteria; Pow- 

 ers of Resistance of the Organism; Natural and Acquired Immunity; 

 Metschnikoff's Phagocytic Theory; Koch's Rules for the Determination 

 of Pathogenic Bacteria; Inoculation of Animals; Methods of Infection. 



I. METHODS OF TRANSMISSION AND THE SPECIAL QUALITIES 

 OP THE PATHOGENIC BACTERIA. 



WE have already considered the general qualities of the bac- 

 teria and the means adopted to obtain further knowledge of 

 their peculiarities. How far have these means enabled us to ad- 

 vance in our acquaintance with definite, special micro-organisms ? 



We shall take them in a certain succession, and the question is 

 whether we can from any standpoint form a definite classification 

 of the bacteria. 



Attempts to form a system on the basis of their natural history 

 or after the manner of their development are still too much in 

 their infancy to be able to serve for guidance. Nor will we rely 

 on purely outward circumstances or the form (the morphological 

 behavior of the bacteria), since the difference between a bacillus 

 and a micrococcus, for instance, is certainly not a matter of such 

 importance that it should make a wide distinction between the 

 different species. The bacteria interest us chiefly in an etiological 

 point of view, because we have recognized many of them as danger- 

 ous parasites of animal organisms that of man included and as 

 the exciters of a whole series of diseases, and it will be simplest and 

 best to base the arrangement on these considerations. 



On the one side, then, stand all those species which are able to 

 exercise a noxious, disease-exciting agency; on the other, those 

 which have not this capacity and cannot become pernicious to 

 man. The line which is thus drawn between the pathogenic and 

 non-pathogenic bacteria is by no means so clear as might at first 

 be thought. A not inconsiderable number of micro-organisms are 

 already known which generally show quite a harmless character, 

 but which under some circumstances may become pathogenic; and 

 many pathogenic species can be induced, on certain terms, to lay 



