170 Arthropods as Direct Inoculators of Disease Germs 
After the discovery of the causative organism, one of the first 
advances was the establishment of the identity of human plague 
and that of rodents. It had often been noted that epidemics of the 
human disease were preceded by great epizootics among rats and 
mice. So well established was this fact that with the Chinese, 
unusual mortality among these rodents was regarded as foretelling 
a visitation of the human disease. That there was more than an 
accidental connection between the two was obvious when Yersin, 
the discoverer of Bacillus pestis, announced that during an epidemic 
the rats found dead in the houses and in the streets almost always 
contain the bacillus in great abundance in their organs, and that many 
of them exhibit veritable buboes. 
Once it was established that the diseases were identical, the atten¬ 
tion of the investigators was directed to a study of the relations 
between that of rats and of humans, and evidence accumulated to 
show that the bubonic plague was primarily a disease of rodents 
and that in some manner it was conveyed from them to man. 
There yet remained unexplained the method of transfer from rat 
to man. As long ago as the 16th century, Mercuralis suggested 
that house-flies were guilty of disseminating the plague but modem 
investigation, while blaming the fly for much in the way of spreading 
disease, show that it is an insignificant factor in this case. 
Search for blood-sucking insects which would feed on both rodents 
and man, and which might therefore be implicated, indicated that 
the fleas most nearly met the conditions. At first it was urged that 
rat fleas would not feed upon man and that the fleas ordinarily attack¬ 
ing man would not feed upon rats. More critical study of the habits 
of fleas soon showed that these objections were not well-founded. 
Especially important was the evidence that soon after the death of 
their host, rat fleas deserted its body and might then become a pest 
in houses "where they had not been noticed before. 
Attention was directed to the fact that while feeding, fleas are in 
the habit of squirting blood from the anus and that in the case of those 
which had fed upon rats and mice dying of the plague, virulent plague 
bacilli were to be found in such blood. Liston (1905) even found,, 
and subsequent investigations confirmed, that the jflague bacilli 
multiply in the stomach of the insect and that thus the blood ejected 
was richer in the organisms than was that of the diseased animal. 
It was found that a film of this infected blood spread out under the 
body of the flea and that thus the bacilli might be inoculated by the 
bite of the insect and by scratching. 
