SECTION II 



INFECTION AND IMMUNITY 



CHAPTER XI 



FUNDAMENTAL FACTORS OF PATHOGENICITY AND INFECTION 



When microorganisms gain entrance to the animal or human body, 

 and give rise to disease, the process is spoken of as infection. 



Bacteria are ever present in the environment of animals and human 

 beings and some find constant lodgment on various parts of the body. 

 The mouth, the nasal passages, the skin, the upper respiratory tract, the 

 conjunctivae, the ducts of the genital system, and the intestines are 

 invariably inhabited by niimerous species of bacteria, which, while sub- 

 ject to no absolute constancy, conform to more or less definite charac- 

 teristics of species distribution for each locality. Thus the colon organ- 

 isms are invariably present in the normal bowel, Doderlein's bacillus 

 in the vagina. Bacillus xerosis in many normal conjunctivae, and staphy- 

 lococcus, streptococcus, various spirilla, and pneumococcus in the mouth. 

 In contact, therefore, with the bodies of animals and man, there is a large 

 flora of microorganisms, some as constant parasites, others as transient 

 invaders; some harmless saprophytes and others capable of becoming 

 pathogenic. It is evident, therefore, that the production of an infection 

 must depend upon other influences than the mere presence of the micro- 

 organisms and their contact with the body, and that the occurrence of 

 the reaction — for the phenomena of infection are in truth reactions be- 

 tween the germ and the body defenses — is governed by a number of 

 important secondary factors. 



In order to cause infection, it is necessary that the bacteria shall gain 

 entrance to the body by a path adapted to their own respective cultural 

 requirements, and shall be permitted to proliferate after gaining a foot- 

 hold. Some of the bacteria then cause disease by rapid multiplication, 

 progressively invading more and more extensive areas of the animal 

 tissues, while others may remain localized at the point of invasion and 



181 



