The deaths of approximately a third of the Native American population during Hernando de Soto's exploration in the 1530s and early 1540s can be attributed primarily to the introduction of Old World diseases, such as smallpox, measles, and influenza, to which Indigenous peoples had no immunity. These epidemics spread rapidly among Native American communities, decimating populations even before direct confrontations with European explorers and settlers.
Additionally, violent encounters and conflicts with de Soto's expedition, which sought to conquer and acquire wealth, contributed to the decline in the Indigenous population. The combination of disease, violence, and disruption of traditional ways of life resulted in significant population loss and social upheaval among Native American tribes during and after de Soto's journey through the southeastern United States.