666 MICROBIOLOGY OF DISEASES OF MAN AND DOMESTIC ANIMALS 



Water-borne Infections. Pure infections of this type occur in prac- 

 tically only five diseases, namely, Asiatic cholera, typhoid fever, 

 paratyphoid fever, and in dysentery of the amoebic and bacillary forms. 

 The length of time that these microorganisms will remain alive in the 

 water depends on the quantity and quality of organic matter present. 

 Only under rare circumstances do bacteria proliferate in water of 

 very high organic matter content. Ordinarily microorganisms will 

 live only a few days if the water is absolutely pure. They have, how- 

 ever, been known to live for several weeks in ordinary river water. The 

 drinking of water or of fluids or material contaminated by water is 

 the common but not the only way these diseases are acquired. 



Infections from Soil. The soil as a source of infectious micro- 

 organisms is of importance in only a few diseases, namely, anthrax, 

 tetanus, symptomatic anthrax, malignant edema, emphysematous 

 gangrene, Asiatic cholera, and typhoid fever. In the first five men- 

 tioned infection always takes place through some wound usually in 

 the skin and in the last two diseases mentioned infection is usually 

 through the intestinal tract but may also occur by means of wounds. 

 The microorganisms of anthrax, tetanus and emphysematous gangrene, 

 or more specifically the spores, will remain in soil for long periods of 

 time. They are sometimes found in the active vegetative stage but 

 it is probable that they do not proliferate to any extent in the soil. 

 They exist as ordinary saprophytes. The microorganisms of typhoid 

 and cholera have been known to remain alive for a year or more in 

 soils containing large quantities of organic matter. The various 

 pyogenic micrococci are also occasionally found in the soil and may 

 enter the body of man and animals through wounds. These last- 

 mentioned organisms may live for indefinite periods of time on the 

 skin and enter the body only when the resistance of some tissue is 

 lowered. 



Infection from Food. Quite a large variety of pathogenic micro- 

 organisms have been found in the various food products. Milk is 

 perhaps the most common food product to be infected. The causal 

 agents of diphtheria, scarlet fever and some other diseases have been 

 disseminated by means of milk. Milk contaminated by water con- 

 taining B. typhosus may be the means of conveying typhoid fever, and 

 the dissemination of Malta fever is accomplished by the drinking of the 

 milk of infected goats. Typhoid fever has also been known to have 



