Many of the members of De Soto's expedition were carriers of diseases such as measles, smallpox, and chicken pox. Very few of American Indian have resistance to the disease because they never exist near their environment before.
As a result, the Infections ran rampant in the American Indian's community.
Since they did not have a medical knowledge to deal with it, the contagion resulted in a lot of deaths. Massively decreased American Indian's population.
