70 ABVANCED PHYSIOLOGY 



The flavors of butter and, in part, of cheese are due to bac- 

 teria. Butter makers have learned that they can make 

 better butter if they put bacteria into the cream out of which 

 butter is to be made. They plant bacteria in cream, much as 

 cooks plant yeast in bread for bread raising, or farmers 

 plant grass in the fields. We must, therefore, look on bacteria 

 in many cases as our best friends. When we remember that 

 we have always carried millions of them in our mouths, and 

 still have enjoyed good health, we must not be alarmed if 

 we are told that there are bacteria in water, or milk, or in the 

 air we breathe. 



Harmful Bacteria. Unfortunately some kinds of harmful 

 bacteria live and grow in the \arious organs of our bodies. 

 It is easy to understand that when these minute parasites 

 are growing in great numbers in different parts of the body, 

 they may produce trouble. Such troubles are called diseases, 

 and to the bacteria which cause them is given the name of 

 disease germs, or pathogenic bacteria. 



Not all kinds of disease are produced by bacteria. Some f 

 like malaria, are caused by tiny animals. The causes of some 

 diseases have not yet been determined, but probably many 

 are brought about independently of bacteria or parasitic 

 animals. Nevertheless, most of the common illnesses are pro- 

 duced by them, and none of the diseases caused by bacteria ever 

 make their appearance unless the germs that cause them 

 first get into the body and find opportunities for growth 

 there. Any disease caused by living germs is called infectious. 



In most of these diseases the bacteria, after having developed 

 in the body for a while, begin to be given off in some way. 

 Sometimes this takes place through the mouth, sometimes 

 through the excreta, sometimes through the breath, sometimes 

 through special discharges of the skin. When bacteria are 

 thus given off from the body of a sick person, they are alive 

 and active, and are generally in condition to enter the body 

 vf another individual and produce the same trouble in him. 



