8 Insects and Disease 



such as typhoid, cholera, tuberculosis and many 

 others, are caused by the presence of bacteria in the 

 body, and it is just as definitely known that such 

 maladies as malaria and sleeping sickness are 

 caused by animal parasites. 



Then there is a long list of other epidemic 

 diseases, such as smallpox, measles and scarlet 

 fever, the exact cause of which has not been de- 

 termined. Many of these are believed to be due 

 to micro-organisms of some kind, and if so they 

 will almost certainly sooner or later be found. 

 Curiously enough most of the diseases in this last 

 class and many of those in the first are contagious, 

 while all that are caused by animal parasites are, 

 as far as is known, infectious but not contagious. 



INFECTIOUS AND CONTAGIOUS DISEASES 



It is important that we keep in mind this distinc- 

 tion. By contagious diseases are meant those that 

 are transmitted by contact with the diseased per- 

 son either directly, by touch, or indirectly by the 

 use of the same articles, by the breath or effluvial 

 emanations from the body or other sources. Small- 

 pox, measles, influenza, etc., are examples of this 

 group. By infectious diseases are meant those 

 which are disseminated indirectly, that is, in a 

 roundabout way by means of water or food or 



