1 82 ELEMENTS OF PHYSIOLOGY 



Typhoid fever, for instance, is usually contracted through 

 the use of water from an impure source. There have 

 been cases of a spring or well becoming infected with the 

 typhoid germs and giving typhoid fever to the unhealthy 

 members of every family that used water from it. It 

 reqtdres many bacteria to produce infection ; to cause some 

 diseases several millions are necessary. Certain kinds of 

 communicable diseases can be caught only by coming in 

 contact with the sick person, or with some article that he 

 has touched, or by going into the room where he is sick. 

 These diseases are said to be contagious, since they are 

 caught by actual contact. Diseases that are transmitted 

 through the air, contact being unnecessary, are said to be 

 infectious. Measles, scarlet fever, mumps, whooping cough, 

 and smallpox are contagious diseases, and germs are sup- 

 posed to be necessary for their existence, but no germ has 

 ever been found. 



315. It has never been proved that germs will cause 

 harm or disease to a perfectly healthy body. The germs 

 that produce boils enter the skin at some scratch or break 

 that reaches the lymph spaces. But a person with pure 

 blood and lymph could have a number of breaks in the 

 skin and would probably not have a single boil. Some 

 germs seem to change their nature or the nature of the 

 toxins they form, according to the material upon which 

 they live. Many germs seem to be harmless when living 

 in certain conditions, but, if they find a diseased condition, 

 they seem to change their nature and begin the forma- 

 tion of poisonous toxins. This is known to be true of the 

 colon bacillus and other germs. 



316. How Bacteria are destroyed within the Body. The 

 body of one who is worthy of physical health has three 

 very efficient means of defense : 



First. The white blood cells have it for their chief 

 function to seek out bacteria and to devour and destroy them 



