350 ECONOMIC ZOOLOGY AND ENTOMOLOGY 



of the same articles. Smallpox, measles and influenza are 

 examples of this group. By infectious diseases are meant those 

 which are disseminated by contaminated water or food, or other 

 infected substances introduced into the body in some way. 

 Typhoid, malaria, yellow fever, and cholera, and others are 

 examples of this class. Thus it is evident that all of the con- 

 tagious diseases may be infectious, but most of the infectious 

 diseases are not as a rule contagious, although some of them 

 may become so under favorable conditions. 



Just one example will show the importance of knowing 

 whether a disease is contagious or infectious. Until a few years 

 ago it was believed that yellow fever was highly contagious, 

 and every precaution was taken to keep the disease from 

 spreading by keeping the infected region in strict quarantine. 

 This often meant much hardship and suffering and always a 

 great financial loss. We now know that it is infectious only 

 and not contagious, and that all this quarantine was unnec- 

 essary. The whole fight in controlling an outbreak of yellow 

 fever, or in preventing such an outbreak, is now directed against 

 the mosquito, the sole agent by which the disease can be trans- 

 mitted from one person to another. 



Definition of a Parasite. A parasite is a living organism 

 that lives in or on some other organism from which it derives 

 its nourishment for the whole or a part of its existence. As a 

 general thing we are accustomed to think of a parasite as work- 

 ing more or less injury to its host. As a matter of fact the num- 

 ber of parasitic organisms that are seriously detrimental to 

 the welfare of their hosts is comparatively small while the 

 number of parasites that do no appreciable harm, and of whose 

 existence in the body the host is often not even aware, is large. 



DISEASES CAUSED BY AMCEB^E 



Amoebic Dysentery. A species of Amoeba, A. dysenteric?, is 

 found associated with a disease of man known as amosbic 

 dysentery. This disease occurs most commonly in tropical 

 countries, but is sometimes met with in temperate zones. 

 The amoebae are found in the alimentary canal, usually in 



