328 BIOLOGY AND HUMAN LIFE 



infection. To protect human life it is necessary either to exterminate 

 some of our neighbors or to see that they keep well. 



One of the special health problems that arose during the World War 

 had to do with the transmission of trench fever, a disease that caused a 

 great deal of suffering and incapacity, although it was seldom fatal. Vol- 

 unteers from the ambulance and field hospital units allowed the blood of 

 patients to be injected into their veins. In this way it was found that 

 the disease is due to a germ too small to be seen with the most powerful 

 microscope. Other volunteers allowed themselves to be bitten by lice 

 taken from the bodies of patients ; many of these developed the disease, 

 while others, bitten by lice from healthy men, remained unaffected while 

 living under exactly the same conditions. These experiments showed that 

 the parasites are carried over by the louse. Measures were then taken to 

 exterminate these insects, and so the disease was brought under control. 



INSECTS IN RELATION TO DISEASE 



1. Characteristics of insects that are related to human disease 



Living things : can become hosts of parasites 



Eating things : attack non-living organic matter (which may breed 

 disease germs) ; attack human beings as weW as other organisms 



Mo\dng things : carry bacteria about on their feet ; carry mi- 

 crobes in the blood and other body juices 



2. Insects as disease-germ carriers (typhoid fly) 



Life history and habits of fly 



Egg, larva, pupa: in manure, decaying food, etc. 

 Adult 



Fhes about freely 



Feeds on all kinds of organic matter 



Visits cesspools, garbage, etc. .then food in kitchen and on table 



Regurgitates materials taken up in one place while visiting 

 another 

 Means of combating fly and protecting health 

 Destroy individual adults (swatting) 



Important in early spring when sur\ivors from previous 



• summer are about to lay eggs 

 Destroy breeding-places 



Screen manure, garbage, etc. ; poison manure etc. 



Prevent scattering of organic refuse 



Screen and poison cesspools etc. 

 Screen houses, sleeping infants, etc. ; protect food 



