Pestis minor is a pathological category that at the height of the third plague pandemic (1894-1959) fueled extensive debate and research among medical scientists. Referring to an attenuated or benign form of plague, evidence of pestis minor or pestis ambulans was produced in medical reports across the world so as to raise the question of whether the disease could survive measures against it by means of temporary transformation. Afflicting its victims only by the slightest lymphatic swellings, this theory went, the disease could thus lurk in the human body until conditions allowed it to break out again in its true, malignant form. This article draws for the first time a history of this contested pathology, the diagnostic and epidemiological questions raised by it, and the way in which it came to play a significant role in debates about the nature of plague at the turn of the nineteenth century.