CHAPTER XXII 



MICEO'OEGANISMS OP THE BODY IN HEALTH AND DISEASE 



Bacteria of the Body. — Many of the bacteria found 

 associated with the bodies of plants and animals are sapro- 

 phytic and are found in these locations by accident. Soil 

 and water contain great numbers of living organisms and 

 many are associated with dust particles in the air. Such 

 organisms, except as they may lead to some confusion in 

 studies of organisms normal to the body, are of no par- 

 ticular importance from the standpoint of health or dis- 

 ease. 



The bodies of animals and many plants harbor what may 

 be termed a normal flora. Many of the organisms compos- 

 ing this flora are commensals, that is, they live upon the 

 waste products of the host and do not in any way cause 

 disease. Some are even decidedly useful. It is probable, 

 for example, that the digestion of cellulose in the alimen- 

 tary tract of ruminants and herbivorous animals in general 

 is due, at least in part, to the activity of certain kinds of 

 bacteria. Still other organisms are true parasites, living in 

 or on the tissues of the body. In a few instances these are 

 known to be helpful. The bacteria, for example, which live 

 in the roots of leguminous plants are truly parasitic, yet 

 they are on the whole helpful and not injurious to the plant. 



Many of the bacteria present in the body are capable of 

 producing disease under certain conditions. Some kinds of 

 organisms have very low degree of virulence or attacking 

 power and under normal conditions are quite incapable of 

 destroying or injuring living tissues. Other forms of bac- 

 teria which occasionally gain admission to the body are 



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