Article: European colonists created treaties with Indigenous American tribes requesting the return of any runaway slaves. For example, in 1726, the British governor of New York exacted a promise from the Iroquois to return all runaway slaves who had joined them. This same promise was extracted from the Huron Nation in 1764, and from the Delaware Nation in 1765, though there is no record of slaves ever being returned. Numerous advertisements requested the return of African Americans who had married Indigenous Americans or who spoke an Indigenous American language. The primary exposure that Africans and Indigenous Americans had to each other came through the institution of slavery. Indigenous Americans learned that Africans had what Indigenous Americans considered 'Great Medicine' in their bodies because Africans were virtually immune to the Old-World diseases that were decimating most native populations. Because of this, many tribes encouraged marriage between the two groups, to create stronger, healthier children from the unions.

Question: Why did natives consider Africans to have a 'Great Medicine'?
because Africans were virtually immune to the Old-World diseases