INSECTS AND HUMAN DISEASE 159 



round the bottom of their trousers as a precaution against 

 being bitten. 



The disease begins with a small blister, as the result of 

 a flea bite, from whence the bacilli make their way to the 

 lymph glands ; and when persons are bitten on the legs the 

 buboes usually appear in the groin. Indian women, on the 

 other hand, usually suffer from buboes in the arm-pit, the 

 reason being that they sweep their floors with their bare 

 hands. It has been proved that male and female fleas can 

 transmit the disease, but the exact method of infection is 

 not perfectly clear. Experiment shows that the blood of 

 a plague flea may contain 100,000,000 bacilli per cubic 

 centimetre, and, as a flea's stomach will hold half a cubic 

 centimetre of blood, it follows that after engorgement on 

 a plague-stricken rat each flea may contain 5,000 bacilli ; 

 moreover, they have the power of multiplying within the 

 insect's body. Into the various theories of the actual 

 infection of man we cannot enter, the most generally 

 accepted is that the faecal matter, which the flea always 

 voids while feeding, as we have explained elsewhere, is 

 carried to the wound made by the insect's bite on the nails 

 of the person bitten. This faecal matter, in an infected 

 flea, is always teeming with Bacillus pestis ; the flea bite 

 sets up irritation, as a relief scratching is resorted to, and, 

 as a result, unconscious infection takes place. 



We have mentioned that the human flea is capable of 

 transmitting plague, and the fact has been proved experi- 

 mentally. Whether it does so naturally is highly improb- 

 able, because the degree of septicaemia in man before death 

 is so much less than it is in rats, that the probability of a 

 human flea imbibing a single bacillus from a plague patient 

 is remote. In the Middle Ages, when the septicaemia was 

 probably much greater, it is possible that the human flea 

 frequently carried the disease from man to man. 



