Role of Fleas in the Transmission of Plague 173 
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 pneu¬ 
monic 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 baccilus were obtained. 
But the great advance which has been made rests on the dis¬ 
covery 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 modem 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 fumi¬ 
gated to destroy the rats and the fleas which they might harbor. 
When anchored at infected ports, ships must observe strenuous 
