It has also been proved that almost all varieties of rat fleas, under favorable circumstances, will bite man and that the most common human flea (_Pulex irritans_) is frequently found upon rats, the flea, generally speaking, being much less particular in his choice of hosts and in his permanence of residence than most insects and ectoparasites in general. Of the rat fleas, _Pulex pallidus_ (_Lœmopsylla cheopis_) is common under various names in India, the Philippines, Australia, Italy, Brazil and in tropical countries generally. It bites both rat and man. _Ceratophyllus fasciatus_, the common rat flea of Great Britain and the United States, also bites both rat and man. In North America and elsewhere certain other fleas of the genus _Ceratophyllus_ have been found upon ground squirrels, cats, rats, sparrows and in chicken yards. Dog fleas and cat fleas (genus _Ctenocephalus_) also infest rats, and fleas of other genera are found upon mice, rats and ground squirrels rather indiscriminately. The significance of these facts in connection with prevention of plague is apparent and it is plain that our warfare against fleas must be made upon _all_ fleas and not upon a single variety. In this connection the possibilities of the conveyance of plague bacilli by other suctorial parasites and by insects which are not parasites, must be borne in mind. Thus the bed-bug, the louse, the tick and the mosquito must be suspected as possible intermediaries and the fly and the cockroach as possible food contaminators. Indeed, laboratory experiments have already incriminated bed-bugs, flies and lice as potential vectors of plague bacilli. Experiment and observation have demonstrated, however, that above all other parasites and insects, the flea is most likely to convey the plague germ from rat to man, by reason of his frequent excursions from rat-host to human-host, his taste for blood from either host, his enormous activity and his ability to jump. After a searching inquiry into the plague question the Indian Plague Commission came to the conclusion that contagion plays a very minor part in the spread of the disease, less than three per cent of human cases being so acquired. This commission also decided that infection is conveyed from rat to rat and from rat to man solely through the agency of fleas. While these conclusions are probably true--and therefore of the utmost importance from the standpoint of practical prevention--I should question whether the other possibilities, however remote, are entirely negligible.