SECTION II 



INFECTION AND IMMUNITY 



CHAPTER XI 



FUNDAMENTAL FACTOKS 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 respira- 

 tory tract, the conjunctivas, the ducts of the genital system, and 

 the intestines are invariably inhabited by numerous species of bac- 

 teria, which, while subject to no absolute constancy, conform to 

 more or less definite characteristics of species distribution for each 

 locality. Thus the colon organisms are invariably present in the 

 normal bowel, Doderlein's bacillus in the vagina, Bacillus xerosis 

 in many normal conjunctivae, and staphylococcus, streptococcus, 

 various spirilla, and pneumococcus in the mouth. In contact, there- 

 fore, with the bodies of animals and man, there is a large flora of 

 microorganisms, some as constant parasites, others as transient in- 

 vaders ; some harmless saprophytes and others capable of becoming 

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

 tion must depend upon other influences than the mere presence of 

 the microorganisms and their contact with the body, and that the 

 occurrence of the reaction for the phenomena of infection are in 

 truth reactions between 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 foothold. Some of the bacteria then cause disease by 



230 



