Plague can be transmitted by the human flea; but it may be doubted whether this often occurs under natural conditions. The rat-fleas seem inclined to take more readily to mankind than the human fleas do to rats. Experiments at Bombay seemed to show that, though the human flea was able to transmit the plague infection, it does not transmit it as readily as the oriental rat-flea. An explanation of this was obtained when it was discovered that _Pulex irritans_ does not live well either on rats, or on guinea-pigs, which were the subjects of the experiments. A count of the fleas was made, each day, in a number of experimental cages, in which live human fleas were placed in company with wild Bombay rats. A great number of human fleas were put into a flea-proof cage along with a rat. Each day a census was taken of the fleas still alive. After twenty-fours hours it was found that little more than one per cent. of the fleas put in could be recovered, and no fleas were ever found alive after the fifth day. The European rat-flea (_Ceratophyllus fasciatus_) seems to be quite as readily able to transmit plague as the oriental insect. How far other fleas are able to transfer infection we have little or no knowledge. But twenty-seven experiments to transmit plague from animal to animal by means of cat-fleas (_Ctenocephalus felis_) were once made and none of these were successful. The reason for the failure we do not know. If infected fleas are kept in captivity after they have fed on septicæmic blood, it is found that, after a while, they are no longer able to convey infection. On being dissected no bacilli are found in them. A clearing process, therefore, evidently goes on. If a number of fleas be fed on a septicæmic rat and, subsequently, be kept under observation and nourished on healthy animals, the proportion found to be infected steadily diminishes day by day. It is remarkable that the existence of numerous plague bacilli in the stomach of a flea does not seem materially to affect the insect’s life. Fleas, in other words, do not suffer from plague though they can transmit it. CHAPTER VIII RAT-FLEAS AND BAT-FLEAS THE chief conclusions arrived at, as the result of the investigations, during the years 1905 to 1909, into the mode of spread of plague in India, may be briefly stated in the following fashion: The Advisory Committee, under whose direction the investigation was carried out, consider that: firstly, in nature, plague is spread among rats by the agency of rat-fleas; secondly, bubonic plague is not directly infectious from man to man; thirdly, in the great majority of cases, during an epidemic of plague, man contracts the disease from plague-infected rats through the agency of plague-infected rat-fleas; fourthly, where there are annual epidemics they occur during some part of that season when the prevalence of fleas is greatest.