Upon taking in plague septicaemia blood the bacilli multiply about the site of the proventriculus as well as distending the oesophagus. This makes an obstruction to the entrance of the stomach resulting in starvation of the flea. This naturally makes the flea more voracious and in the ineffectual muscular efforts to take in blood, regurgitation of the contents of the oesophagus occurs, thereby infecting the person upon whom the flea is trying to feed. This obstruction apparently may be overcome in some way as the plague-infected flea does not necessarily die. Still from lack of sufficient fluid such fleas are liable to be killed off if the relative humidity is low, as in dry weather. Further investigations have shown that the proventricular plug may be channeled, but in such case valve action is lost and the contents of the stomach are regurgitated, thus making such a flea more dangerous than one with an obstructed proventriculus. Such fleas may continue infectious for 67 days. Very interesting in this connection is the fact that Heiser found plague-infected fleas in the desk of a European at Manila who died of plague. A mummified rat was found in one of the drawers of this desk, from which successful animal inoculations for plague were made. Heiser notes that data would indicate that these fleas probably remained alive 2 weeks after the death of the rat which brought about their infection. Another very striking finding during the same Manila plague outbreak (1912-1913) was that bedbugs found on the sleeping mat of a human victim of plague showed bipolarly staining bacilli. Bacot has carried on experiments in which fleas infected two months previously and kept in a cool place could transmit plague. This would indicate the danger from plague-infected fleas which had been held in material packed away in boxes. _Method of Spread._—The spread of plague epizootics among rats seems to be rather by the fierce brown sewer rat, _Mus norvegicus_. The more delicate black house rat, _Mus rattus_, usually receives its infection from the sewer rat. When the rat dies the fleas leave the dead body and seek a new host, preferably one similar to the one just abandoned. The sewer rat reaching the basement of houses and dying of plague is deserted by his fleas. These will attach themselves to the house rats which go from basement to roof of the house and later these dying are abandoned by the fleas which, in the absence of a rodent host, will feed on man and infect him.