From the later 17th century, the Native American population dropped drastically due to warfare and disease. Settlers regularly waged war against Native American tribes in order to place more land under their control and push Native Americans away from their settlements. Additionally (and probably more drastic), Native Americans had not developed the same immunities Europeans had to infectious diseases. Prior to settling in North and South America, Europeans lived in cities rife with disease and filth. While this contributed to a substantially shorter lifespan for Europeans relative to people in more sanitary living conditions, it also led to increased immunity to diseases. Europeans then came to the Western Hemisphere with their diseases, which were then spread (overtly and indirectly) to Native Americans whose more sanitary lifestyle resulted in weaker immunity.
