Answer:
Diseases killed the majority of the Native Indian population in the ancient civilizations that helped Spaniards to conquer the region by establishing colonies in Central and South America.
Step-by-step explanation:
The sharp decline of the native population happened due to the diseases which came with Europeans as they arrive in the New World. The Native Indians were not immune to diseases like chickenpox, smallpox, scarlet fever, cholera, malaria, measles etc. Many died after coming to contact with European while some remained weak. Spanish conquistadors were able to conquer ancient civilization like Inca, Maya, and Aztec because remaining Native Indians were not strong enough to fight.