But, however clear the evidence that fleas are the most important agent in the transfer of plague, it is a mistake fraught with danger to assume that they are the only factor in the spread of the disease. The causative organism is a bacillus and is not dependent upon any insect for the completion of its development. Therefore, any blood-sucking insect which feeds upon a plague infected man or animal and then passes to a healthy individual, conceivably might transfer the bacilli. Verjbitski (1908) has shown experimentally that bed-bugs may thus convey the disease. Hertzog found the bacilli in a head-louse, _Pediculus humanus_, taken from a child which had died from the plague, and McCoy found them in a louse taken from a plague-infected squirrel. On account of their stationary habits, the latter insects could be of little significance in spreading the disease. Contaminated food may also be a source of danger. While this source, formerly supposed to be the principal one, is now regarded as unimportant, there is abundant experimental evidence to show that it cannot be disregarded. It is believed that infection in this way can occur only when there is some lesion in the alimentary canal. Still more important is the proof that in pneumonic plague the patient is directly infective and that the disease is spread from man to man without any intermediary. Especially conclusive is the evidence obtained by Drs. Strong and Teague during the Manchurian epidemic of 1910-11. They found that during coughing, in pneumonic plague cases, even when sputum visible to the naked eye is not expelled, plague bacilli in large numbers may become widely disseminated into the surrounding air. By exposing sterile plates before patients who coughed a single time, very numerous colonies of the bacillus were obtained. But the great advance which has been made rests on the discovery that bubonic plague is in the vast majority of cases transmitted by the flea. The pneumonic type forms a very small percentage of the human cases and even with it, the evidence indicates that the original infection is derived from a rodent through the intermediary of the insect. So modern prophylactic measures are directed primarily against the rat and fleas. Ships coming from infected ports are no longer disinfected for the purpose of killing the plague germs, but are fumigated to destroy the rats and the fleas which they might harbor. When anchored at infected ports, ships must observe strenuous precautions to prevent the ingress of rats. Cargo must be inspected just before being brought on board, in order to insure its freedom from rats. Even lines and hawsers must be protected by large metal discs or funnels, for rats readily run along a rope to reach the ship. Once infested, the ship must be thoroughly fumigated, not only to avoid carrying the disease to other ports but to obviate an outbreak on board.