TEXT-BOOK OF BACTERIOLOGY. 311 



otitis or something else? Does the "sword of Damocles" actually 

 hang at every moment so close to our head, and must it not ap- 

 pear almost miraculous that anybody at all is spared by this ter- 

 rible foe ? We can account for this certainly verj' striking fact 

 only by the circumstance that it requires, as a rule, certain prepa- 

 ratory, as yet unknown factors, to enable this bacterium to make its 

 attacks. The healthy coverings and tissues of the body resist the 

 micro-organisms; only when the native powers of resistance are 

 weakened or neutralized will the foreign invaders take a firm footing 

 and begin their pernicious activity. The better the medium agrees 

 with them, the more rapidly they will develop and the greater will 

 their virulence become. All the minute gradations of their infec- 

 tious power (observed even under the conditions of their natural 

 appearance and surely strong enough to determine the severity of 

 a single case or the character of an epidemic) will find their ex- 

 planations in this hypothesis. 



XVII. DIPHTHERIA BACILLUS (LOFPLER). 



Another kind of bacterium resembles in many respects Fraenkel's 

 diplococcus. 



We conceive genuine pneumonia to be of an essentially uniform 

 character, its origin usually being due to one and the same cause ; we 

 thus distinguish it from those so-called " secondary pneumonias " not 

 unusually developing in the course of other diseases (such as typhoid, 

 small-pox, etc.) and showmg, pathologically, the symptoms of gen- 

 uine lung-inflammation, but being caused by other micro-organisms 

 (presumably by streptococci). 



Diphtheria, another affection of infectious origin, is of a similar 

 nature. We know quite a number of pathological processes occur- 

 ring w^ith the formation of croupous or diphtheritic changes of the 

 mucous membranes and by post-mortem appearances not distin- 

 guishable from the processes accompanying diphtheria proper. 

 The former may, however, arise from various causes, while the lat- 

 ter, as a whole and in every part of its course, exhibits so many 

 peculiarities as an entity, that intelligent investigators have 

 always considered it as a disease by itself and attributed its origin 

 to one and the same cause. As its infectious nature is manifested 

 so decidedly and is transmitted most fearfully from man to man by 

 direct contagion, it can readily be understood that the specific 

 micro-organism has long been looked for. 



All reliable observers having thus far agreed that the blood and 

 the internal organs of persons dying of diphtheria are usually 

 completely free from micro-organisms, the conclusion was reached 



