Answer:
De Soto's expedition spread Eurasian diseases into the interior of the country, inadvertently killing thousands of Native people.
Really Basic Explanation:
Hernando de Soto, obviously a European, traveled to Florida and explored around. While exploring around the Florida area, he came into contact with Native Americans. The Spanish carried a deadly disease called smallpox. Since the Spanish had already come into contact with smallpox before, they developed immunity. Unfortunately for the Indians, none of them had ever experienced a smallpox outbreak in their region. The new introduction to smallpox devastated the Native American population, and in certain tribes over 90% of the Native Americans died.