Until recent years, the plague had not been known to occur in the New World but there were outbreaks in Brazil and Hawaii in 1899, and in 1900 there occurred the first cases in San Francisco. In California there were 125 cases in the period 1900-04; three cases in the next three years and then from May 1907 to March 1908, during the height of the outbreak, 170 cases. Since that time there have been only sporadic cases, the last case reported being in May 1914. Still more recent were the outbreaks in the Philippine Islands, Porto Rico, and Cuba.
On June 24, 1914, there was recognized a case of human plague in New Orleans. The Federal Health Service immediately took charge, and measures for the eradication of the disease were vigorously enforced. Up to October 10, 1914 there had been reported 30 cases of the disease in man, and 181 cases of plague in rats.
The present-day methods of combating bubonic plague are well illustrated by the fight in San Francisco. Had it not been for the strenuous and radical anti-plague campaign directed by the United States Marine Hospital Service we might have had in our own country an illustration of what the disease can accomplish. On what newly acquired knowledge was this fight based?
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.