DISEASE, INFECTION, AND RESISTANCE 275 



Certain infectious diseases of insects are also of economic 

 importance. The best known of these are the flaeherie of 

 the silk worm, American foul brood and European foul 

 brood of bees. 



Contagious and Noncontagious Diseases. — A contagious 

 disease is one which is more or less readily transmissible 

 from one individual to another by direct or indirect con- 

 tact. A Tioncontagious disease is one that is not thus trans- 

 mitted. In practice the term contagious is usually modi- 

 fied. We speak of certain diseases as being highly con- 

 tagious, of others as being moderately or slightly contagious. 

 At the other end of the series we have diseases which are 

 noncontagious. Particular notice may be drawn to the 

 fact that the terms infectious and contagious are frequently 

 confused in popular literature and discussion. It will be 

 noted that infectious refers to cause, contagious to method 

 of spread. 



Factors Predisposing to Disease. — The resistance of an 

 individual to a disease undoubtedly varies from time to 

 time. It is probable that unusual fatigue, hunger, unusual 

 chilling or excessive heating of the body may increase the 

 susceptibility of the individual. Certain diseases are more 

 apt to attack children, others more frequently attack adults. 

 There are also diseases characterized usually as diseases of 

 old age. 



Heredity and Disease. — To be inherited, in the strict 

 sense of the term, a disease must be of a character such that 

 it can be transmitted indefinitely from one generation to 

 the next. Certain defects, such as color blindness, may be 

 transmitted in this fashion, but this is not true of any of 

 the infectious diseases. Animals may at birth occasionally 

 be infected with certain diseases acquired by them from the 

 mother before birth. Such diseases are, however, not in- 

 herited in the strict sense of the term. 



