40 INFECTIOUS AND PARASITIC DISEASES. 



processes now in use in the arts and manufactures 

 are but devices to harness these natural forces. Thus 

 in the manufacture of hnen, hemp and sponges, in the 

 butter, cheese, and vinegar industries, in tobacco- 

 curing, etc., bacteria play the important role. 



Curiously, in the popular mind, bacteria 

 Saprophytes, are only associated with disease, and are 



regarded with abhorrence on that account ; 

 yet of the untold hundreds of species, only about forty 

 are known to produce disease in human beings. 



Bacteria which are useful to mankind in so many 

 ways belong to the class of organisms known as obligate 

 saprophytes, i.e., "an organism that lives upon dead 

 organic matter." Bacteria that produce disease, on 

 the other hand, belong to the parasites and infectious 

 agents, a description of which has already been given 

 in the preceding chapter. However, the ability of a 

 bacterium to live upon dead organic matter does not 

 determine its status from the standpoint of the produc- 

 tion of disease. The obligate saprophytes, as their 

 name imphes, are restricted to a diet of dead things; 

 but the parasitic bacteria, while preferably parasitic 

 in their habits, may nevertheless lead a saprophytic 

 existence. Siich species are known as jacultative 

 saprophytes. Most of the disease-causing bacteria 

 belong to this class, a factor that has proven of incal- 

 culable value in studying them. Indeed, bacteriology 

 dates its rapid progress from the time that pathogenic 



