The basis was laid in 1894, when the plague bacillus was first discovered. All through the centuries, before and during the Christian era, down to 1894, the subject was enveloped in darkness and there had been a helpless, almost hopeless struggle in ignorance on the part of physicians, sanitarians, and public health officials against the ravages of this dread disease. Now its cause, method of propagation and means to prevent its spread are matters of scientific certainty. 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 attention 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 modern 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 attacking 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.