INSECTS AND MANKIND 437 



various flies, and most notably in fleas. A good summary 

 account of the progress of the research, in which Japanese, 

 French, and Russian students co-operated, has been given 

 by H. Russell (1913), and it has been well established that 

 the rat-flea Xeiiopsylla cheopsis is the most frequent carrier 

 of Bacillus pestis. The stomachs of fleas which have 

 sucked blood from plague-stricken rats may become densely 

 congested with masses of the bacilli. If these fleas get on 

 human subjects, the micro-organisms are voided by the 

 fleas on the skin, and their bites, or the abrasion of the 

 skin by their tormented victims' endeavour to get relief by 

 scratching, open a way for the entrance of the germs into 

 the human system. Fleas may also infect one human 

 being from another, but for the continual spread of bubonic 

 plague it seems necessary that there should be an association 

 between the four organisms : bacillus, rat, flea, and man. 

 When this connection is understood it is seen at once how 

 the elimination of plague follows improved conditions of 

 housing, sanitation, and cleanliness. It is, however, im- 

 portant to remember the danger which arises from the fact 

 that the number of rats in Great Britain approaches or 

 equals the total human population, that Bacillus pestis has 

 been detected in rats in various parts of England, and that 

 the common rat-flea Ceratophyllus fasciatus may serve as a 

 transmitter. 



In many parts of Europe typhus has often proved a 

 scourge well-nigh as deadly as plague, and has been especially 

 fatal in the crowded, unhealthy environment of oldtime 

 camps and prisons. The part played by two species of lice 

 (Pediculus capitis and P. corporis) infesting human beings 

 in transmitting the micro-organism of typhus was demon- 

 strated in 1910 by H. T. Ricketts and others. Conse- 

 quently when during and after the Great War serious 

 epidemics of typhus appeared in Eastern Europe it was 

 realised that louse-free persons would be safe from infection, 

 so that the medical officers of various armies and relief 

 expeditions were able to take the necessary measures for 

 enforcing that cleanliness which is a condition of safety. 



