The historic period pertains to the Native Americans since their first contact with the Europeans. Tribes such as the Cheyenne, Sioux, and Cherokee are all descendants of the ancient people from the earlier time periods. So what happened to the great empires of the Mississippian Period? What become of the city at Spiro Mounds, the pottery of the Caddo and Quapaw? While theories have speculated everything from natural disasters to the depletion of land resources, modern day anthropologists place the downfall of these prehistoric civilizations on the diseases brought over from Europe. Some demographers estimate that as many as nineteen out of twenty Native Americans died of European diseases like smallpox, influenza and tuberculosis.