Parasitism and Disease 



9 



other substances taken into or introduced into the 

 body in some way. Typhoid, malaria, and yellow 

 fever, cholera and others are examples of this class. 

 Thus it is evident that all of the contagious dis- 

 eases may be infectious, but many of the infectious 

 diseases are not as a rule contagious, although some 

 of them may become so under favorable con- 

 ditions. 



Just one example will show the importance of 

 knowing whether a disease is contagious or in- 

 fectious. 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 con- 

 tagious, and that all this quarantine was unneces- 

 sary. 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 transmitted 

 from one person to another. 



EFFECT OF THE PARASITE ON THE HOST 



We have seen how a few parasites in or on an 

 animal do not as a rule produce any appreciable 



