Answer:
Native American populations declined rapidly due to a lack of immunity to disease
Explanation:
At that time, many Europeans who came to American continent carried out viruses that could cause diseases such as smallpox, influenza, and measles within their body.
When they "Encountered" The native Americans, these viruses passed on to the Natives and infected them.
Because the natives have lack of immunity to disease (they never experienced diseases like that in their life) , the effect to their body is much greater compared to the Europeans. This led to large amount of death among Native Americans.
