CHAPTER X 

 OTHER INSECTS THAT TRANSMIT DISEASE GERMS 



The insects other than house flies and mosquitoes that trans- 

 mit disease germs are principally blood-sucking flies, fleas, bed- 

 bugs, and lice. The relations of many of these insects to disease 

 are very little known, but in a few years we may expect the jury 

 of scientists either to convict or acquit those now under indict- 

 ment. 



Fleas and Bubonic Plague. — The connection between fleas 

 and bubonic plague is now well known. This disease is caused 

 by a very small bacterium which causes fever, glandular swellings, 

 and often death. Many epidemics are recorded in history; in 

 the sixth century about half of the people in the Roman Empire 

 died of it; in India from 1901 to 1904 it caused about two mil- 

 lion deaths ; and in China, Egypt, South Africa, and in our own 

 seaports epidemics have occurred or have been threatened. 

 Careful studies of the plague have proved that the bacteria 

 causing it are chiefly carried from diseased rats to man by a kind 

 of flea which is now known as the plague flea. 



Control of Plague in San Francisco. — In the neighborhood 

 of San Francisco the California ground squirrels have also be- 

 come diseased by plague germs that have been transferred to 

 them by rat fleas. The spread of the disease throughout North 

 America through the agency of ground squirrels, rats, and fleas 

 is thus made possible. 



" During the last few years San Francisco has been fighting 

 an outbreak of plague that in other days would have been 

 nothing less than a national calamity. But with modern 

 methods of handling it, based on knowing what it is, what causes 



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