BACTERIA IN DISEASE. 157 



CHAPTER V. 



BACTERIA IN DISEASE. 



To the physician and the student of medicine the study of 

 bacteriology is interesting chiefly on account of the great 

 importance attributed to bacteria in producing disease. 

 The presence in an organism of one or a number of organ- 

 isms of another species, which flourish as parasites upon 

 the first, is a phenomenon of very wide occurrence in na- 

 ture. It is, in fact, nearly universal. It may be observed 

 among plants as well as animals, for example in the familiar 

 galls seen on some of the higher plants, and mostly caused 

 by the larvae of insects harbored by the plant. We also 

 find animals, such as tape-worms and the trichina spiralis, 

 living as parasites upon other animals. The functions of 

 the bacteria make them peculiarly suited to leading a para- 

 sitic existence. The fact that they possess no chlorophyll, 

 and that they are therefore unable to form carbon com- 

 pounds from the carbon dioxide of the atmosphere, makes 

 it necessary for them to secure such compounds from pre- 

 existing organic matter. Many of them, furthermore, 

 flourish better when they are able to obtain nitrogenous 

 food from organic matter rather than from inorganic salts 

 containing nitrogen. Most bacteria find the necessary nutri- 

 ment in the dead bodies of other animals and plants ; they 

 constitute what are known as saprophytes. But some of 

 them flourish upon the living bodies of other plants and 

 animals in whom they may produce disease. 



The phenomena of disease, we shall find, are very largely 

 due to the numerous waste products of the activities of 

 bacteria, which act as poisons to the host. 



