STABILITY OF VIRULENCE OF _B. Pestis_.--According to Strong, stability of virulence is a marked characteristic of _B. pestis_, it having been shown by him that it is difficult to increase the virulence of a very virulent strain or to intensify an attenuated one in laboratory animals, working with monkeys, rats and guinea-pigs.[2] If his observations are correct (and they seem to correspond with the findings of other observers), the oft-recorded occurrence of a preponderance of mild cases of plague in the early days of an epidemic and the gradual subsidence in intensity of the disease as the epidemic approaches its close will have to be explained upon other grounds than those of variability of virulence by attenuation of virulent strains alone. While he admits that _B. pestis_ may become attenuated under certain conditions many times during the course of an epidemic, it may also regain its virulence, he contends, under other conditions. [2] "Studies in Plague Immunity," R. P. Strong, Philippines Journal of Science, June 1907, No. 3. Frequent reference has been made to these studies in the preparation of this article, for which acknowledgment is hereby made. With these facts concerning the cause and the manner of extension of plague and its menace before us, we are in position to approach the problem of prevention intelligently, and in the case of plague prevention is preëminently preferable to cure, as well as decidedly more practicable. I think we may be permitted here to sum up the problem of plague prevention thus: Without fleas, without rats, or without human plague cases, there can be no extension of plague, practically speaking. Therefore the destruction of both rats and fleas, the isolation of human plague cases, and the exclusion from them of all suctorial parasites and insects, will provide practical security for mankind generally. A word concerning pneumonic plague may be permissible. This form of plague occasionally occurs in epidemics of great fatality, as, for example, the epidemic in Manchuria, North China, a few years ago. The mystery of this outbreak was largely dispelled by the work of the Americans, Strong, Teague and Barber, of the Bureau of Science of Manila. The occurrence of secondary pneumonia in bubonic or septicæmic plague is rather common and it is likely that such secondary plague pneumonias are the starting points of epidemics of pneumonic plague, _i.e._, of cases of primary plague pneumonia, the point of infection being in the respiratory organs and the infection being acquired through the inspiration of plague bacilli. The principal prerequisites seem to be an extremely moist atmosphere under confined conditions and a low temperature; conditions most unfavorable to evaporation and ventilation. Under these conditions the pneumonic patient sprays plague bacilli into the air while coughing and droplet infection follows.