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 im- 

 portance attributed to bacteria in producing disease. The 

 presence in an organism of one or a number of organisms of 

 another species, which flourish as parasites upon the first, is a 

 phenomenon of very wide occurrence in nature. 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 har- 

 bored by the plant. We also find animals, such as tapeworms 

 and the trichina spiralis, living as parasites upon' other animals'. 

 The conditions favorable to the growth of certain bacteria 

 make them peculiarly suited to leading a parastic existence. 

 The fact that they possess no chlorophyll, and that they are 

 therefore unable to form carbon compounds from the carbon 

 dioxide of the atmosphere, renders it necessary for them to 

 secure such compounds from pre-existing organic matter. 

 Most 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, those 

 known as saprophytes, find the necessary nutriment in the 

 dead bodies of other animals and plants; but some of them, 

 those known as parasites, flourish upon the living bodies of 

 other plants and animals and produce disease. 



The phenomena of disease, as has been well established, are 

 due in a number of cases to the numerous waste products of the 

 activities of bacteria, which act as poisons to the host. 



167 



