INSECTS AND DISEASE 375 



The yellow-fever mosquito is a domestic insect. It is seldom 

 found far from human habitation and for this reason can be 

 comparatively easily controlled, for if every possible breeding 

 place within a few hundred feet of every house was properly 

 cared for this mosquito would no longer be present. 



Fleas and Plague. Plague is a disease that for ages was 

 regarded as one of the greatest of human scourges, because 

 when once started in a region there seemed to be no way to stop 

 its ravages. But it, too, has been conquered within the last 

 decade by the discovery of the cause of the disease, and the 

 way -in which it is transmitted from one host to another. 



Plague is primarily a disease of rats and other rodents, and it 

 is commonly spread by the fleas with* which these animals are 

 always infested. When a rat dies of the plague the fleas which 

 have been sucking its blood leave it for some other host. As 

 most of the rat-fleas will also bite man the infected fleas may 

 communicate the disease to him. 



The disease is caused by the presence in the blood of a 

 bacillus, Bacillus pestis. There are several types of plague 

 distinguished by the way in which they affect the patient, and 

 they are probably spread in different ways. The bubonic 

 plague is the most common. It may occur as sporadic cases 

 among animals and be transferred from them to man by fleas. 

 Under favorable conditions this type may become epidemic. 

 Pneumonic and septicaemic plague are more virulent types, and 

 may be transmitted directly from man to man and probably 

 also conveyed by insects. 



In the efforts to control the last outbreak of plague that 

 occurred in the United States the whole fight was directed 

 against the rats that had the disease and against the fleas that 

 carried the infection to man. Hundreds of thousands of rats 

 were killed, their breeding places destroyed, warehouses and 

 storerooms made rat-proof, covered garbage cans were required 

 in every yard, and many other measures were taken to make it 

 impossible for rats to exist in any great numbers in the infested 

 region. In this way there was stamped out in a remarkably 

 short time an epidemic which might at an earlier time have 

 proved a national calamity. In some places ground-squirrels 



