30 



INSECTS AND HUMAN WELFARE 



without question an utterly unwarranted assumption for 

 it has undoubtedly been broupjht to Europe from some little- 

 known quarter of the globe, unless it nuiy have previously 

 existed in Europe which does not appear probable. During 

 the later part of the war it was successively recognized as a 

 distinct disease, suspected of association with the louse, and 

 soon proved actually to be louse-borne. We now know that 



Fig. 13a. Map illustrating the distribution of typhus fever in the old world. It is a common disease 

 in the regions marked with black, less common in those marked with cross-hatched lines and rare 

 in those marked with parallel lines. (After Byam, et ol.) 



the disease is due to a living microorganism probably of such 

 small size that it cannot be recognized under the microscope. 

 This virus is obtained by the lice with their meal of blood 

 taken from an infected person. At least five days must elapse 

 before the louse becomes capable of transmitting the disease, 

 indicating that the organism must undergo a development of 

 definite periodicity in the insect. If it is transferred to 

 another person its bite is not or only rarely infectious, but 

 its excrement contains the virus and if scratched into the 

 skin, trench fever develops. Typhus fever is a very danger- 



