CHAPTER VII. 



THE RELATIONS OF MICRO-ORGANISMS TO DISEASE- 

 INFECTION AND IMMUNITY. 



The Occurrence of Bacteria and Other Micro-organisms in 

 the Body : its Protective Mechanism. 



BACTERIA are invariably present in greater or less numbers in the 

 mouth, nose, upper air passages, gastro- intestinal and genito- urinary 

 tracts of men and animals. ' Into these places they are more or less con- 

 stantly brought by the respired air, 2 by food and drink, and in other 

 ways. But common and often abundant as are these germs upon the 

 external and internal surfaces of the body, they do not often pass through 

 the healthy mucous or cutaneous surfaces, so that under normal condi- 

 tions the tissues, the viscera, and the circulating fluids are germ-free. 3 



Except for certain pathogenic forms which may, under unsanitary 

 conditions, have been set free and transported from men or animals 

 suffering from infectious disease, the bacteria upon the cutaneous or 

 mucous surfaces of the body are for the most part harmless ; while cer- 

 tain intestinal forms may even be useful in promoting digestion. 



Certain bacteria which do not often and some which never induce 

 disease, find in or upon the human body such favorable conditions for 

 their existence that they are commonly present there. 



The body is guarded in various ways from the incursions of patho- 

 genic and other bacteria, which may be commonly present or only occa- 

 sionally lodged upon its surfaces. Among the protective agencies of the 

 body may be mentioned the firm, dense skin which while intact protects 

 the interior from the entrance of almost all known micro-organisms ; the 

 epithelial investment of the mucous membranes in several places swept 

 by cilia ; the protected situation of most of the mucous surfaces, the ger- 

 micidal qualities of some of the secretions, such as the gastric juice, 

 mucus, etc. 4 



1 For a summary of facts concerning the bacterial flora of the body surfaces consult 

 Welc7i, "Surgical Bacteriology," "System of Surgery by American Authors, " Dennis. 

 See also Ford, "The Bacteriology of Healthy Organs," Trans. Assoc. Amer. Phys., vol. 

 xv., p. 389, 1900, Bibl. 



2 For a resume of micro-organism in the air see Gottstein, Lubarsch and Ostertag's 

 "Ergebnisse," Jahrgang iv., pp. 87 et seq., 1897; also Firth, "Studies from the Depart- 

 ment of Pathology," Col. Phys. and Surg., Columbia University, vol. vii. 



3 It should be remembered that the gastro-intestinal canal, the lungs, and other 

 viscera which are in communication with the exterior, although within the limits of the 

 body, still form, strictly speaking, its outside, in distinction from the intimate recesses 

 of the tissues in which the life processes go on. 



4 For a resume of the protective action of the skin, mucous membranes, etc., see 

 MetscJmikoff, "Immunity in Infectious Disease's," Trans., 1904. 



