



varieties, will, on ooeasiots, aoli as para^ 

 sites to man, and will bite vigorously. 

 The common European rat flea, or cera- 

 tophyllus fa8ciatu8,on the otuerhand, differs 

 in important structural respects, and can, 

 only with great difficulty, be induced to feed 

 on man. Does it not appear to be 

 something more than a coincidence 

 that where we find a ratflea closely allied 

 to the human flea, there we have plague 

 amongst rats as well as man, whereas in 

 places where the ratflea varies widely 

 from the human flea the occurrence of 

 epidemic rat-plague is, at any rate, not an 

 important phenomenon. 



The theory was for long held that the 

 plague baccilus lives and multiplies in 

 the earth, but out of innumerable at- 

 tempts at its recovery from the earth- 

 floors of plague-infected Eastern hovels, 

 iiot a single success has resulted under 

 natural conditions. As in the case of 

 Yellow fever, it has been stated to have 

 been spread by infected clothing. It pro- 

 bably is so spread, but in some other way 

 than the mere bacillary infection of such 

 clothing, for the plague bacillus has never 

 been recovered from infected clothing, un- 

 less in cases of gross experimental con- 

 tamination. Plague is particularly a 

 disease of locality, and especially tends to 

 infect those who sleep in such a locality, 

 sparing those who move actively about 

 during their visits and sleep or rest else- 

 where. It chooses dark, squalid, vermin- 

 haunted vicinities, and avoids to a great 

 extent airy, well-lit places. The experience 

 of plague hospitals is a curious one, for 

 whilst, in ancient days, even a short visit 

 to a pest-house was attended with great 

 danger, it is rare for an attendant in a 

 modern plague-hospital to be attacked. 

 That the disease itself has not varied from 

 ancient times we know from contemporary 

 records, but whilst the modern hospital 

 is especially built with a view to cleanli- 

 ness, light and airiness, the ancient hos- 

 pitals seem to have combmed all the 

 hygienic offences of its day. To sum up we 

 have certain facts which seem to point in 

 a particular direction. Plague does not, 

 apparently, thrive in the soil, nor is it 

 known to enter the body with the food, in 

 human cases at least. In the ordinary 

 form— excluding plague pneumonia— it 

 enters by the skin, and in the bubonic 

 form it certainly enters in the lymphatic 

 area drained by the first affected 



gland, Wfttei? play* no parfc ia 

 its dissemination. Meteorological fac- 

 tors have no influence except in 

 one significant indirect instance— that 

 in India the plague mortality is noticed to 

 rise materially after a cold night, or a 

 heavy fall of dew, and especially after 

 rains during the dry season, whereby 

 people are driven into their homes instead 

 of sleeping outside as usual. Clothing is 

 apparently capable of conveying the 

 disease ; but the bacillus is not recover- 

 able from clothing. And lastly, a marked 

 association of the rat with plague out- 

 breaks has been noticed since remote 

 periods in parts of the world where the 

 rat flea closely resembles the human flea, 

 and no such association where the rat 

 flea shows, at any rate at the present day, 

 wide structural differences from the human 

 flea. 



This circumstantial evidenca appears 

 to incriminate the insect, but circum- 

 staniial evidence is not enough. Dr. 

 Simond, in 1897, had proved that 

 by infecting into a mouse broth in 

 which fleas had been emulsified, after 

 having fed on a plague-stricken animal, 

 plague could be produced in that 

 mouse. This experiment was success- 

 fully repeated on several occasions by my- 

 self and others, and the results obtained 

 were sufficiently encouraging to induce 

 further work. Fleas of certain species 

 were fed on plague-infected animals, and 

 after varying periods of starvation were 

 allowed to bite healthy rats under condi- 

 tions which excluded any probability qf 

 the disease being contracted otherwise 

 than by flea bite. These rats died of un- 

 deniable plague, and it was found that the 

 flea could convey plague in this . way up 

 to at least three days after a meal of infected 

 blood. There is no reason why it should 

 not do so for very much longer. It was 

 found that in one case, at least, this con- 

 veyance from rat to rat was effected by a 

 human flea. In other cases the rat-pulex 

 was used. A further experiment was 

 performed in which plague was conveyed 

 from a human being to two rats by means 

 of fleas. This latter experiment was of 

 especial significance, since, on subse- 

 quent examination, it was found that 

 while three of the insects used for the 

 purpose were the pulex palladus of rats, 

 one was a pulex irritans, or human flea. 

 By cutting the insects in seotions after a 



