38o THE ANIMALS AND MAN 



among which larger animalculae are darting, are bacteria. 



Of the many kinds of bacteria some are useless to man, 

 some are of benefit, while many are positively dangerous. 

 Those found in the scum are probably harmless and may 

 even be beneficial as scavengers. 



Bacteria feeding upon substances such as meat, fish, 

 milk, etc., decompose or rot them. 



The importance of bacteria in the economy of our lives is 

 so great that it has given rise to a study called "bacteriology." 



Beneficial bacteria. — When the farmer plows his field, 

 he turns under the soil the dry grasses from the top surface. 

 These are then attacked by certain bacteria which decom- 

 pose them and set free the chemical compounds of which they 

 are composed. These substances, mostly originally derived 

 from the soil, are thus again returned to the soil to enrich it 

 for the use of growing plants. The manufacture of cheese 

 and butter would be impossible without the aid of certain 

 bacteria. 



Harmful bacteria. — For many years now, owing chiefly to 

 the work of Pasteur and Koch, it has been known that several 

 diseases that inflict mankind are caused by the parasitic 

 growth of certain microorganisms in the human body. 

 These harmful microorganisms break down or poison the 

 blood or other body tissues and thus greatly derange the 

 normal functions. The derangement results in disease 

 and even death. It is of importance, therefore, for every- 

 one to know how to prevent the spread of such diseases 

 that result from the attacks of bacteria. These are the 

 infectious and contagious diseases such as tuberculosis, 

 typhoid fever and diphtheria. Malaria and certain other 

 diseases are known to be caused by Protozoan microbes. 

 The cure of patients having these or other diseases 

 lies for the most part in the hand of the physician, 

 but the prevention of the diseases lies chiefly in the hands 

 of the individual and the community. The best rule for the 



