How Insects Cause or Carry Disease 55 



louse on rats (HcBmatopinus spinulosus) is respon- 

 sible for the spread from rat to rat of a certain 

 parasite. (Trypanosoma lewisi), which, however, 

 does not produce any disease in the rats, but if 

 they are capable of acting as alternative hosts 

 for such parasites, it is quite possible that they 

 may also carry disease-producing forms. 



HOW INSECTS MAY CARRY DISEASE GERMS 



Insects may carry the germs or parasites which 

 cause disease in a purely mechanical or accidental 

 way, that is, the insect may in the course of its 

 wanderings or its feeding get some of the germs on 

 or in its body and may by chance carry these to 

 the food, or water, or directly to some person who 

 may become infected. Thus the house-fly may 

 carry the typhoid germs on its feet or in its body 

 and distribute them in places where they may enter 

 the human body. 



Several other flies as well as fleas, bedbugs, 

 ticks, etc., may also carry disease germs in this 

 mechanical way. While this method of transmis- 

 sion is just as dangerous as any other, and possi- 

 bly more dangerous because more common, an- 

 other method in which the insect is much more 

 intimately concerned is more interesting from a 

 biological standpoint at least and will be discussed 



