The history of Dysentery, as told by the younger Heberden, has been a favourite instance of the steady decrease of a disease in London during the 18th century. I have shown the error in this, and at the same time have proved from the London bills of mortality of the 17th and 18th centuries that Infantile Diarrhoea, which is now one of the most important causes of death in some of the great manufacturing and shipping towns, was formerly still more deadly to the infancy of the capital in a hot summer or autumn. Asiatic Cholera brings us back, at the end of the history, to the same great problem which the Black Death of the 14th century raised near the beginning of it, namely, the importation of the seeds of pestilence from some remote country, and their dependence for vitality or effectiveness in the new soil upon certain favouring conditions, which sanitary science has now happily in its power to withhold. I have left Influenza to be mentioned last. Its place is indeed unique among epidemic diseases; it is the oldest and most obdurate of all the problems in epidemiology. The only piece of speculation in this volume will be found in the five-and-twenty pages which follow the narrative of the various historical Influenzas; it is purely tentative, exhibiting rather the _disjecta membra_ of a theory than a compact and finished hypothesis. If there is any new light thrown upon the subject, or new point of view opened, it is in bringing forward in the same context the strangely neglected history of Epidemic Agues. Other subjects than those which occupy the nine chapters of this volume might have been brought into a history of epidemics, such as Mumps, Chickenpox and German Measles, Sibbens and Button Scurvy, together with certain ordinary maladies which become epidemical at times, such as Pneumonia, Erysipelas, Quinsy, Jaundice, Boils and some skin-diseases. While none of these are without pathological interest, they do not lend themselves readily to the plan of this book; they could hardly have been included except in an appendix of _miscellanea curiosa_, and I have preferred to leave them out altogether. It has been found necessary, also, to discontinue the history of Yellow Fever in the West Indian and North American colonies, which was begun in the former volume.