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To the Editor:
—Preceding or accompanying an outbreak of human plague, a similar outbreak occurs among rodents. A mild epidemic may suddenly be transformed into a more severe type, or a severe one may be reverted into a milder form. Mixed types occur, depending on individual susceptibility. Observations made in Egypt deal with the mechanism involved in such transformations, the rôle of the bacteriophage, the mechanism by which rats acquire a natural immunity to the disease without visible symptoms, and the sudden disappearance of severe symptoms in those having the illness (abortive cases). The latter fact has been proved to be due to the presence of a bacteriophage, thus confirming the general statement of d'Herelle that the bacteriophage is concerned in recovery.Furthermore, it has been found that, prior to an outbreak in rats, little variation in individual susceptibility is seen, while during the epidemic great differences are noted and
Doorenbos W. NATURAL VACCINATION AND ITS INFLUENCE ON THE COURSE OF EPIDEMICS, WITH PARTICULAR REFERENCE TO BUBONIC PLAGUE. JAMA. 1928;91(6):416. doi:10.1001/jama.1928.02700060050030