6 Introduction 



mode and seat of invasion of the organism. But these are compara- 

 tively few in number. The bacillus of tuberculosis belongs to this 

 minority. Whether it enters subcutaneously, by the eye or by the 

 respiratory, digestive or genito-urinary passages, it invariably pro- 

 duces tubercular lesions more or less grave and more or less capable 

 of generalisation. On the other hand, a very large number of micro- 

 organisms only exert their pathogenic action when they invade the 

 organism at definite points. The anthrax bacillus, when introduced 

 through the slightest lesion of the skin or of the mucous membranes, 

 produces in man, and in a large number of mammals, a very grave 

 and usually fatal disease ; when absorbed in the vegetative state with 

 food, it is almost always innocuous. With the cholera vibrio we 

 have an exactly opposite condition of affairs. When inoculated, even 

 in large numbers, below the skin in the human subject, it rapidly 

 disappears, producing merely insignificant disturbances ; but when 

 the same vibrio is introduced into the digestive canal it develops and 

 produces Asiatic cholera, a disease so often terminating in death. 



All these variations and peculiarities associated with the nature of 

 infective agents are of great importance from the point of view of 

 immunity. 



Do diseases come from without or do their causes arise within the 

 organism? is a pressing question, long discussed by pathologists. 

 Those who have discovered most of the pathogenic micro-organisms 

 have ranged themselves on the side of the former hypothesis. For 

 the majority of them the essential etiological factor in the causation 

 of infective diseases consists in the invasion of the patient by the 

 pathogenic micro-organism from the outer world. This theory is in 

 perfect harmony with many of the admitted facts of epidemiology, 

 according to which the viruses of the most deadly epidemic diseases, 

 such as Asiatic cholera, yellow fever, and bubonic plague, must be 

 imported into a country previously free from the disease before an 

 epidemic can be developed. In anthrax and trichinosis it is recognised 

 [7] that the parasites must come from without. Hence, in the study of 

 pathogenic micro-organisms one always follows the rule that it is 

 essential to find the specific micro-organism in all cases of the disease 

 in question and to prove its absence in healthy individuals or in 

 those who are affected with other diseases. Thus, Koch ', in his classical 

 researches on Asiatic cholera, insisted on the fact that the cholera 

 vibrio was always found in cases of this disease but never in healthy 

 1 Deutsche med. Wchnschr., Leipzig, 1884, SS. 499, 519. 



