36 INSECTS AND HUMAN WELFARE 



bacilli are preserved for nearly a week at least and sometimes 

 fully a month; and there is actually an increase in their num- 

 ber during the first few days. Infection from these insects 

 may then occur through their bites, if they contain extremely 

 virulent bacilli, but probably occurs more commonly by the 

 insects being crushed in situ after they have punctured the 

 skin. 



There are many reasons for believing that infantile paralysis 

 may be an insect-borne disease, and a few words in regard to 



JAN FEB MCH APL MAY JUN JUL AUQ SEP OCT NOV DEC 



FIG. 17. Seasonal Incidence of Infantile Paralysis in the United States During 1916 



this baffling disease may not be amiss. Its summer preva- 

 lence (Fig. 17) is well known and its general distribution and 

 occurrence are similar to those of insect-borne diseases. 

 That it may be proved to be spread by the rat-flea is not 

 improbable, and if so, would be another strong argument for 

 the reduction of our rat population. 



No account of insect-borne diseases, however brief, could 

 be complete without some reference to animal diseases. A 

 few of these have already been referred to incidentally as 

 affecting both man and animals, and it is quite likely that 

 other human diseases whose etiology is at present obscure, 

 will in the future be shown to bear some relation to those of 



