East Prussia
Approximately one-third of East Prussia's population died in the plague and famine of 1709–1711, including the last speakers of Old Prussian. The plague, probably brought by foreign troops during the Great Northern War, killed 250,000 East Prussians, especially in the province's eastern regions. Crown Prince Frederick William I led the rebuilding of East Prussia, founding numerous towns. Thousands of Protestants expelled from the Archbishopric of Salzburg were allowed to settle in depleted East Prussia. The province was overrun by Imperial Russian troops during the Seven Years' War.

Q: What wiped out one third of East Prussia's population during the early 1700's?
the plague and famine