CHAPTER V. 
SAPROPHYTISM, PARASITISM AND PATHOGENISM. 
I. Definitions and Limits. 
II. The Cycle of Parasitism. 
III. The Cycle of Pathogenism. 
IV. Distribution of Parasitic and 
Pathogenic Bacteria in Nature. 
V. How Parasitic and Pathogenic 
Bacteria Reach Man. 
A. The Occurrence of Parasitic 
Bacteria upon the Bodies of 
Healthy Men and Animals. 
B. How Pathogenic Bacteria Reach 
the Body. 
1. Air-borne Infection. 
(a) Dust. 
(b) Droplet. 
2. Soil-borne Infection. 
3. Water-borne Infection. 
4. Food-borne Infection. 
5. Animal Carriers. 
(a) Direct Contact. 
(b) Indirect Transfer. 
(c) Mechanical Transfer. 
(d) Intermediary Host. 
6. Human Carriers. 
7. Contact Infection. 
VI. 
8. Germinal and Prenatal 
Infection. 
C. Portal of Entry: Atria of Inva- 
sion. 
1. Skin and Adnexa: Ear, 
Eye, Subcutaneous Tis- 
sue, Tonsils, Salivary 
Glands, Nasal Cavity, 
Lungs. 
2. Mucous Membranes: 
Mouth, Stomach, Intes- 
tines. 
3. Genito - urinary System: 
Vagina, Uterus, Urethra, 
Urinary Bladder and 
Ureter, Kidneys. 
D. Where Bacteria Multiply in the 
Body. 
E. Where and How Bacteria Es- 
cape from the Body. 
Balanced Pathogenism; Epidemi- 
ology. 
Seasonal and Annual Distribution 
OF Pathogenic Bacteria. 
Epidemiology. 
I. DEFINITIONS AND LIMITS. 
The most conspicuous and important function of bacteria in the 
economy of Nature is to maintain a continuity between the animal 
and vegetable kingdoms by restoring in utilizable forms to the plant 
world the elements contained in the complex organic compounds 
which comprise the dead bodies of plants, animals and their pro- 
ducts. Bacteria dissipate much of the energy acciunulated in these 
dead bodies and oxidize the elements contained in them to inorganic, 
fully mineralized salts. These salts are resynthesized by the chloro- 
phyll-bearing plants through the energy of sunlight to carbohydrates, 
proteins and fats, and in these complex combinations the elements 
are again available for animal food. 
The bacteria which live upon this dead organic matter, and whose 
function it is to effect its degradation and ultimate mineralization, 
are called sayroyhytic bacteria. They are specifically the most 
numerous, chemically the most active, and economically the most 
important members of the phylum Bacteriaceae. They are rarely 
pathogenic, that is, they rarely initiate disease in man or the lower 
animals. Whenever they are found associated with morbid processes 
