NEW DISEASES : THE NATURE OF INFECTIONS 23 



The Nature op the Infections and of Pathogenic Microbes 



We must take as our first postulate that every infectious 

 disease is brought about by the growth within the body, and 

 diffusion throughout the system, of toxines * of one or other 

 particular species of micro-organism. This being so, it is deserv- 

 ing of note that these pathogenic microbes do not form an order 

 by themselves, but, on the contrary, are singularly diverse in 

 their affiliations. Still influenced by the work of the last quarter 

 of the nineteenth century, we are apt to forget this, and classing 

 the whole subject under the heading of pathogenic bacteriology, 

 to overlook this notable diversity. How few remember that a 

 generation before pure cultures of any bacteria were obtained 

 and tested, Schoenlein demonstrated the relationship and the 

 constant presence of a mould in Favus. How few again recall 

 Goodsir's work of the 'forties and his association of a Sarcina 

 with gastric atony. Flagellate and ciliate protozoa, sporozoa 

 and yet other genera of protozoa, forms like the spirochaetes, 

 which certainly are not bacteria, but still are not surely classified, 

 seeming to be midway between protozoa and protophytes : 

 filterable viruses which, if protophytic, are evidently of more 

 than one order and, again, are not bacteria as generally accepted, 

 even if, according to Hort, some of them represent a phase in 

 the life - cycle of certain forms included among the bacteria. 

 With scarce an exception every genus of micro-organism has 

 its representative or representatives among the pathogenic 

 microbes, or, put otherwise, every pathogenic microbe has closely 

 related forms or species differing from it in little beyond the fact 

 that the one is virulent, the other non-virulent. 



On Allied Species of Virulent and Non- virulent Microbes 



Next it is to be noted that these allied species are found 

 suggestively growing in the cavities or on the mucous surfaces 

 of the body, in the same habitat as the virulent forms, or again 

 in water and food-stuffs which are taken up by the individual. 



1 There is to my knowledge only one clear-cut example of an organism 

 which, growing upon one of the surfaces of the body without penetration into 

 the tissues, sets up disease by the diffusion inwards of its toxines — the Bacillus 

 botulinus, which is responsible for one form of meat-poisoning. Strictly 

 speaking, this organism induces an intoxication and not an infection. 



