The “Schwarze Tod,” or “Black Death,” was published in 1832; and I was prompted to undertake its translation, from a belief that it would prove interesting at a moment when another fearful epidemic, the Cholera, with which it admitted of comparison in several particulars, was fresh in the memory of men. The “Tanzwuth,” or “Dancing Mania,” came out shortly afterwards; and, as it appeared to me that, though relating to a less terrific visitation, it possessed an equal share of interest, and, holding a kind of middle place between a physical and a moral pestilence, furnished subject of contemplation for the general as well as the professional reader, I determined on adding it also to our common stock of medical literature. When the “Englische Schweiss,” or “Sweating Sickness,” which contained much collateral matter little known in England, and which completed the history of the principal epidemics of the middle ages, appeared in 1834, I proceeded to finish my task; but failing in the accomplishment of certain arrangements connected with its publication, I laid aside my translation for the time under a hope, which has at length been fulfilled, that at some future more auspicious moment, it might yet see the light. It must not be supposed that the author, in thus taking up the history of three of the most important epidemics of the middle ages, although he has illustrated them by less detailed notices of several others, considers that he has exhausted his subject; on the contrary, it is his belief, that, in order to come at the secret springs of these general morbific influences, a most minute as well as a most extended survey of them, such as can be made only by the united efforts of many, is required. He would seem to aim at collecting together such a number of facts from the medical history of all countries and of all ages, as may at length enable us to deal with epidemics in the same way as Louis has dealt with individual diseases; and thus by a numerical arrangement of data, together with a just consideration of their relative value, to arrive at the discovery of general laws. The present work, therefore, is but one stone of an edifice, for the construction of which he invites medical men in all parts of the world to furnish materials[1].