42 INFECTION AND IMMUNITY. 



anus bacillus and that of malignant edema) re- 

 tain life and virulence for a long time. It is still 

 uncertain whether the latter proliferate in this sit- 

 uation, or whether they persist merely through the 

 agency of their resistant spores. Inasmuch as they 

 are anaerobic in character, their life may in some 

 instances be prolonged through symbiotic aerobic 

 organisms, which surround them and create for 

 them an atmosphere which is poor in oxygen. The 

 presence of many saprophytes is unfavorable to 

 the life of certain micro-organisms in the soil (ty- 

 phoid, plague). In so far as is known none of 

 the protozoa which are pathogenic for man occur 

 in the soil naturally. 

 water, and 2. Water, milk and other food substances rarely 

 harbor important pathogenic organisms under nat- 

 ural conditions. On the other hand, they play a 

 very important part in carrying such microbes 

 from man to man, and in some instances from 

 animals to man, as explained in succeeding chap- 

 ters. Stagnant water frequently sets up acute enter- 

 itis, which may in some instances be due to its 

 chemical constituents and in others to saprophytic 

 organisms. 



Solid foods, as fruits and vegetables, may occa- 

 sionally harbor bacteria, particularly when in a 

 state of decay, which have moderate pathogenic 

 powers for the intestinal tract; or, by providing 

 suitable alimentation or otherwise modifying the 

 contents or resistance of the alimentary tract, may 

 render organisms virulent which otherwise would 

 be harmless. 



Meats, healthy in the first instance, are subject 

 to invasions by micro-organisms from the intes- 

 tinal tract, after the death of the animals (fowls, 



