CHAPTEE IV. 



SOURCES OF PATHOGENIC MICRO-ORGANISMS. 



(1) Earth, Etc.; (2) Food Substances; (3) Ani- 

 mals; (4) Body Surfaces of the Individual. 



Micro-organisms which produce disease in man 

 may be derived (1) from the earth, other inani- 

 mate material, or from vegetable growth; (2) 

 from water, milk or other food substances; (3) 

 from animals, directly or indirectly; (4) from the 

 body surfaces of the individual himself; (5) from 

 other human beings, and (6) from insects. 



1. The extent to which the superficial earth is Earth. 

 contaminated with micro-organisms depends on 

 various conditions, particularly the presence of 

 dead organic matter, moisture, temperature, the 

 degree of exposure to light and sunlight, the chem- 

 ical composition of the soil, and the admixture of 

 animal excretions. Some of them inhabit the soil 

 naturally and are comparatively harmless sapro- 

 phytes, and may even be of great value in the 

 regeneration of soils. Others, of pathogenic char- 

 acter, seem to occur naturally in the earth or on 

 vegetation, where they multiply readily (the pyo- 

 genic cocci, actinomryces). On the other hand, a 

 large number of pathogenic organisms reach inan- 

 imate nature only as they are deposited with the 

 excretions of man (those of typhoid, paratyphoid, 

 dysentery, cholera, plague, tuberculosis, etc.), or of 

 animals (tetanus bacilli). Many, as the organisms 

 of typhoid, cholera and plague, probably do not 

 proliferate at all in the earth, although some (tet- 



