9, INFECTION AND RESISTANCE 



presented after all a problem infinitely more simple than that of the 

 infection of living tissues with bacteria. For, given any organic 

 material containing suitable nutritive constituents, with favorable 

 environmental conditions of moisture and temperature, and spon- 

 taneously or experimentally inoculated with germs of a proper 

 species, and the phenomena which ensued were merely those of bac- 

 terial growth, in which an active part was played by the bacteria 

 only, the dead organic materials serving simply as a passive men- 

 struum for these activities. 



During the earlier days of the development of bacteriology, 

 therefore, when the attention of investigators was concentrated pri j 

 marily upon the discovery of the specific causal agents of various in- 

 fectious diseases, it seemed that the simple bringing together of 

 pathogenic germ and susceptible subject should suffice for the ac- 

 complishment of an infection. We have learned, however, that the 

 process is much more involved, and that, fortunately for the sur- 

 vival of the higher animals and man, the conditions which determine 

 infection are intimately dependent upon a variety of secondary 

 modifying factors. 



Throughout nature bacteria are abundant, and the environment 

 of man and animals, the outer integuments of skin and hair, and the 

 mucous membranes of the conjunctiva?, the intestinal and respira- 

 tory tracts, are constantly inhabited by a thriving bacterial flora. 

 The distribution of certain species in definite localities is often suffi- 

 ciently constant to be regarded as a normal condition. Thus the 

 Bacillus xerosis is a characteristic inhabitant of the conjunctiva, 

 certain cocci and spirilla are always present in the mouth and 

 pharynx, as is Doderlein's bacillus in the vagina. The fact that 

 bacilli of the colon group are invariably present in the bowels of ani- 

 mals and man from the first few days or hours after birth has even 

 been interpreted by some investigators as a physiologically beneficial 

 condition. In the course of ordinary existence, therefore, and much 

 more so during the course of accidental exposure to individuals in 

 whom infection is present, the bodies of the higher animals are in 

 intimate contact, not only with ordinarily harmless bacteria (sapro- 

 phytes), but also with many varieties of the micro-organisms spoken 

 of as "pathogenic" or disease-producing. Perfectly normal indi- 

 viduals have, then, on occasion, been found to harbor diphtheria 

 bacilli in nose and pharynx, meningococci have been found in simi- 

 lar localities, and tetanus bacilli, the bacillus of malignant edema, 

 the Welch bacillus, and other distinctly pathogenic germs have been 

 isolated from the intestinal contents of individuals who showed no 

 evidence of disease. In fact, the problem of the so-called bacillus car- 

 riers persons who, though themselves apparently well for the time 

 being, harbor within their bodies and distribute to their environ- 

 ment bacteria capable of causing disease in others is, as we shall 



