BACTERIA 283 



with a high-powered microscope. On this account, they 

 escaped detection until recently, and the ravages due 

 to them were ascribed to mysterious or supernatural 

 causes. 1 By means of the microscope, aided by modern 

 methods of investigation, we now recognize many of 

 these organisms and associate them with the diseases 

 which they cause. 



Bacteria, protozoa and molds. Each well-defined 

 infectious disease is in all probability caused by its 

 special microscopic organism, or microbe, although in 

 some diseases the particular kind has not as yet been 

 discovered. The microbes which come from the vegeta- 

 ble kingdom and are minute one-celled plants of vary- 

 ing forms are called bacteria. Those which come from 

 the animal kingdom and also consist of single cells of 

 varying forms, are known as protozoa. 2 In addition 

 to bacteria and protozoa, certain higher forms of vege- 

 table and animal life are responsible for disease. They 

 are chiefly certain kinds of molds and various forms 

 of animal parasites, as the tapeworm and trichina. 



Exposure to disease. To many of these varying 

 kinds of disease-producing microbes we are more or less 

 exposed at all times. They may be present in the dust 

 of the air which we breathe, on the food which we eat, 

 or in the water which we drink. They may lurk upon 

 the skin and in the various cavities of the body. In 

 many cases, however, they are harmless because of the 

 body's ability to destroy them as fast as they find en- 

 trance to it. When the body has been weakened, how- 

 ever, or when the invading microbes are very numerous 

 and virulent, they are able to invade it and to multiply 



1 Defoe's "History of the Plague" gives a vivid picture of the old 

 attitude toward infection. 



2 Literally, "first animal." 



