What disease was a major cause of the downfall of the indigenous
populations of South America?

Repeated outbreaks of Old World infectious diseases such as influenza, measles and smallpox (to which they had no natural immunity) were the main cause of depopulation.

Explanation:

in 550 b.c.e. cyrus the great, the leader of the persians, conquered the medes and united the iranian people under one ruler for the first time. cyrus became the first king of the persian empire and went on to establish one of the largest empires in world.