Answer:
What caused Soto's disease was the crossing of the Mississippi River.
Step-by-step explanation:
Hernand de Soto was a famous and influential Spanish explorer and conqueror who had a strong influence on the American continent, even reaching the territories of North America, being the European explorer to find the Mississippi River.
As you may already know, the misssisssipi river is one of the widest and longest in the territory where the USA is today. For this reason, the crossing of de Soto and his men to this river was very laborious, difficult and exhausting which made De Soto sick, with severe fevers, dying soon after. He chose Moscoso Alvarado to replace him as leader of the expedition, which made Moscoso famous.