In seeking for the causes of its decline and extinction we must keep prominently in view the fact that the virus was brought into the country from abroad as the Black Death of 1348-9. But for that importation it is conceivable that there would have been no signal history of plague in Britain. Its original prevalence was on a great scale, and there were several other widespread epidemics throughout the rest of the 14th century. In the first volume of this history I have collected evidence that plague was endemic or steady for long periods of the 15th and 16th centuries in London, with greater outbursts at intervals, and that in the 17th century it came chiefly in great explosions. Something must have served to keep the virus in the country, and more especially in the towns, until at length it was exhausted. An exotic infection, or one that had not arisen from indigenous conditions, and would probably never have so arisen, does not remain indefinitely in the country to which it is imported. Thus Asiatic cholera, imported into Europe on six, or perhaps five, occasions in the 19th century, has never become domesticated; and yellow fever had a career in the southern provinces of Spain during some twenty years only. Plague did become domesticated for about three centuries in England, and for longer in some other countries of Europe; but it died out at length, and it would almost certainly have died out sooner had it not found in all European countries some conditions not altogether unsuited to it. What were the favouring conditions?