BACTERIA AND DISEASE 319 



infected with bacteria; in fact they often gain access 

 when it is thought that they are effectively excluded. 

 They are especially liable to be carried about on particles 

 of dust. Look at a beam of light entering a room and 

 you will usually see myriads of small bodies floating in 

 the air. For every particle that you can see there are thou- 

 sands that are too small to be visible. When we remem- 

 ber that even invisible particles may be over a million times 

 as large as a bacillus we can gain some idea of what it 

 means to effectively exclude the possibility of bacterial 

 infection. 



While many bacteria are harmful, most species are proba- 

 bly beneficial. The r61e of bacteria in causing decay 

 is one of great importance. Organisms are resolved back 

 into their primitive constituents, and their atoms may live 

 again in the bodies of other forms of life. On account of 

 their causing the decomposition of organic material the 

 bacteria play an important part in the production of soil; 

 Certain species associated with the roots of plants have 

 the property of utilizing the nitrogen of the air and con- 

 verting it into a form that may be subsequently used by 

 plants and animals. Many of the processes of fermenta- 

 tion, such as the souring of milk and the production of 

 vinegar, are the result of bacterial activity. In fact were 

 it not for these invisible organisms higher forms of life 

 would not be able to exist in the earth. 



That certain species of bacteria should come to live in 

 the tissues of plants and animals is quite analogous to the 

 fact that many other organisms have adopted a parasitic 

 mode of life. The disease-producing bacteria do not wait 

 until an organism is dead before they attack it, but like 

 the bacteria that effect the decomposition of the dead 

 body they tend to bring about the dissolution of the 

 organism. Louis Pasteur, the great French investigator 



