FERMENTATION AND GERM DISEASES 71 



Hence one person can easily take a disease from another. 

 Such diseases are called contagious. 



All contagious diseases such as diphtheria, typhoid fever, 

 tuberculosis, measles, whooping cough, scarlet fever and small- 

 pox seem to be produced by living organisms, minute in size 

 but capable of living a parasitic life in the body, and passing 

 readily from one person to another. The germs causing them 

 are not always of that class called bacteria, but they are always 

 minute, always parasitic and may all be properly classed 

 as disease germs. The methods by which these germs pass 

 from the patient to the healthy individual are various. The 

 bacteria themselves are not capable of any considerable 

 amount of motion, and are never able, of themselves, to travel 

 from person to person. They must be carried, and the various 

 kinds are carried by different means. Some are carried in the 

 air, some by water and some by insects. 



Some diseases are very contagious, by which we mean 

 that they are very easily " caught "; and others are slightly 

 contagious. This really refers to the ease with which the germs 

 can be carried from person to person and taken into the body. 

 In cases where the germs must pass into the water and be sub- 

 sequently drunk before they can enter the body of another 

 individual, it is evident that the liability of contagion is less 

 than in the cases where they simply pass into the air and are 

 breathed in by a second individual. All such diseases are 

 infectious, but not all are necessarily contagious. A study of 

 methods of transmission will give us valuable information as to 

 ways of preventing contagion. 



IMMUNITY 



Almost all kinds of bacteria are harmless even if they do 

 get into our bodies. This means that they are not able to 

 multiply in the body so extensively as to produce trouble. 

 We are immune against them. Moreover, certain kinds of 

 bacteria can grow in some of the lower animals and produce 



