Bacteria and Fungi 253 



clear, but 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 supphes 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 

 toxins interfere with the normal working of the bodily processes 

 and cause illness. The body xmder these circumstances pro- 

 duces substances called antitoxins, which protect the tissues 

 until the bacteria are destroyed by leucocytes (colorless 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 blood contains 

 substances that kill the germs. Habits of personal cleanli- 

 ness, sleeping in the open air, and careful diet help to increase 

 immunity against many of the common diseases to which we 

 are subject. 



Some of the commoner bacterial diseases are tuberculosis, 



