Introduction 7 



persons. Almost simultaneously Loeffler 1 , in the course of his work 

 on the etiology of diphtheria, demonstrated the presence of a specific 

 bacillus not only in a large number of cases of this disease but also in 

 the throat of a healthy child; and this fact at first prevented him 

 from accepting this bacillus as the real cause of diphtheria. 



This view accepted by two such eminent bacteriologists cannot 

 however be maintained. It is impossible to assume that each time 

 that a pathogenic micro-organism makes its way into a susceptible 

 species its presence must inevitably be followed by the production of 

 the specific disease. Although the discovery by Loeffler of the 

 diphtheria bacillus in the throat of healthy individuals has repeat- 

 edly been confirmed, it is impossible to doubt the etiological role of 

 this organism in diphtheria. Moreover, it has been established that 

 Koch's vibrio, although undoubtedly the etiological factor in the 

 production of Asiatic cholera, has nevertheless been recognised in the 

 digestive canal of perfectly healthy persons. 



As soon as he is born, man becomes the habitat of a very rich 

 microbial flora. The skin, the mucous membranes, and the gastro- 

 intestinal contents become stocked with such a flora, but a very small 

 number of these micro-organisms have up to the present been recog- 

 nised or described. The buccal cavity, the stomach, the intestines 

 and the genital organs offer a feeding ground for Bacteria and 

 inferior Fungi of various kinds. For long it was thought that in 

 healthy individuals all these micro-organisms were inoffensive and 

 sometimes even useful. It was supposed that when an infective 

 malady was set up a specific pathogenic micro-organism was added to 

 the benign flora. Exact bacteriological researches have, however, 

 clearly demonstrated that as a matter of fact the varied vegetation in 

 healthy persons often includes representatives of noxious species of [8] 

 bacteria. Besides the diphtheria bacillus and the cholera vibrio, which 

 have repeatedly been found in a virulent form in perfectly healthy 

 individuals, it has been demonstrated that certain pathogenic micro- 

 organisms, e.g. the Pneumococcus, staphylococci, streptococci and the 

 Bacillus coli, are always, or almost constantly, found among the 

 microbial flora of healthy persons. 



This observation has necessarily led to the conclusion that in 

 addition to the micro-organism there exists a secondary cause of 

 infective diseases a predisposition, or absence of immunity. An in- 

 dividual in whom one of the above-mentioned pathogenic species is 

 1 Mitth. ausd. K. Gesundheitsamte, Berlin, 1884, Bd. 11. S. 421. 



