90 THE FLEA [CH. 



The blood of a plague-infected rat may contain an 

 enormous number of plague bacilli. Although such 

 figures do not convey any very clear idea of numbers, 

 as many as a hundred million bacilli have been found 

 in a cubic centimetre of rat's blood. A rat-flea, with 

 a stomach of average size, might receive therefore as 

 many as 5000 germs into its stomach ; and it is clear 

 that fleas feeding on a large proportion of plague- 

 infected rats just before death would be almost 

 certain to imbibe at least some plague bacilli. There 

 is, moreover, good evidence for believing that multi- 

 plication of the plague bacilli may take place in the 

 flea's stomach. Nor does the blood imbibed by the 

 flea cease to be infective when it passes from 

 the stomach. Both the contents of the rectum and 

 the excrements of fleas taken from plague rats often 

 contain abundant and actively virulent plague bacilli. 

 A number of infected fleas are put into a test-tube : 

 the mouth of the tube is covered with a glass slide, 

 and the mouth is turned upside down. The fleas are 

 then seen to run over the slide, and, in a short time, 

 they deposit an appreciable amount of fgecal matter 

 on the surface. This under the microscope is seen to 

 be covered with plague bacilli ; and a large percentage 

 of guinea-pigs, who have an emulsion of the ftecal 

 matter injected into them, contract plague. 



It is remarkable that, so far as we know at 

 present, the plague bacillus is confined to the flea's 



