6 



Insects and Disease 



is, they actually repel them and do not suffer 

 them to exist in or on their bodies, or they are 

 tolerant toward them. In the latter case the 

 parasites live at the expense of the host, but the host 

 has become used to their being there, adapted to 

 them, and the injury that they do, if any, is neg- 

 ligible. 



But when a new animal comes into the region 

 from some other locality the parasites may be ex- 

 tremely dangerous to it. There are many strik- 

 ing examples of this. Most of the people living 

 in what is known as the yellow fever belt are im- 

 mune to the fever. They will not develop it even 

 under conditions that would surely mean infection 

 for a person from outside this zone. Certain of 

 our common diseases which we regard as of little 

 consequence become very serious matters when in- 

 troduced among a people that has never known 

 them before. The cattle of the southern states 

 are immune to the Texas fever, but let northern 

 cattle be sent south or let the ticks which transmit 

 the disease be taken north where they can get on 

 cattle there, and the results are disastrous. 



Another striking example and one that is at- 

 tracting world-wide attention just now is the 

 trypanosome that is causing such devastation 

 among the inhabitants of central Africa. With the 



