Bacteria and Their Relations to Life 391 



removes from it disease-producing bacteria. The most dreaded 

 of all the water-borne diseases is typhoid fever, and the cities 

 are now much freer from this disease than are the country 

 districts where people depend upon well water. Surveys in some 

 of the Middle Western states showed that from one fifth to one 

 third of the wells examined contained large numbers of bacteria 

 derived from surface drainage. In such wells there is always 

 danger that the surface waters may bring in disease-producing 

 bacteria, especially typhoid germs derived from human sources. 



Other sanitary practices, such as quarantine, disinfection, 

 admitting plenty of sunshine into living rooms, cleaning walls 

 and floors, removing dust, cooking food, washing and scald- 

 ing dishes, pasteurizing milk, and keeping food supplies in 

 refrigerators, are all related to the control or elimination of 

 bacteria. 



Bacteria and disease. When certain bacteria grow in the 

 body, they produce poisonous substances called toxins. These 

 interfere with the normal working of the bodily processes and 

 cause illness. The body under these circumstances produces 

 substances called antitoxins. These are substances formed by 

 the cells of the body, which neutralize the effects of the toxins, 

 either by combining with them chemically, or by rendering the 

 cells insensitive to the toxins. In this way they protect the 

 tissues until the bacteria are destroyed by leucocytes (color- 

 less blood corpuscles) or in other ways. Not all persons are 

 equally susceptible to infectious diseases. A person may be 

 immune to a disease because his blood contains the corresponding 

 antitoxin or is able to produce it, or because his body is insensitive 

 to the bacterial toxins. Some of the commoner bacterial diseases 

 are tuberculosis, pneumonia, grippe, diphtheria, typhoid fever, 

 colds, lockjaw, and " blood poisoning." 



A fundamental fact that should be learned in this connection 

 is that no one can contract a bacterial disease unless he comes in 

 contact with the particular bacterium which causes that disease. 



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