CHAPTER I 



INSECTS AND THE PUBLIC HEALTH 



SCARCELY more than two decades have elapsed since the 

 scientific world entertained its first suspicion that certain 

 human diseases might be spread through the agency of in- 

 sects. Eighteen years have gone by since that suspicion be- 

 came an established fact, and in this short space of time 

 so much has been learned concerning the pernicious activities 

 of these small animals in disseminating disease-causing or- 

 ganisms among man and the higher animals, that the science 

 of preventive medicine can now be applied to many impor- 

 tant diseases which were before utterly beyond its reach. 

 Every year brings forth fresh evidence that insects are im- 

 portant factors in relation to public health, and adds to the 

 list of diseases that are partially or entirely dependent upon 

 certain insects for their spread. 



A brief statement of the nature of communicable diseases 

 and of the general habits of the kinds of insects that are impli- 

 cated in carrying disease will serve to define roughly the field 

 of medical investigation which is open to the entomologist. 

 Communicable diseases are invariably due to parasitic or- 

 ganisms in the body which are capable of inducing similar 

 symptoms in other persons or animals if transferred to 

 healthy individuals from diseased ones. Many conditions 

 modify the transfer of communicable diseases ; some individ- 

 uals are more easily infected than others; some may be 

 immune as the result of a previous attack; and, on the other 

 hand, the virulence of pathogenic organisms often varies 

 greatly in accordance with conditions to which they have 

 been subjected previously. A simple method of spread occurs 

 with many diseases, for example typhoid fever and pulmon- 

 ary tuberculosis. With the former, the Bacillus typhosux 



