Final answer:
Native Americans were devastated by diseases like smallpox due to a lack of immunity and previous exposure, with European invaders bringing epidemics that native populations had no resistance against, coupled with societal disruption and malnutrition worsening the impact.
Step-by-step explanation:
The diseases like smallpox affected Native Americans so badly because the indigenous peoples had never been exposed to the infectious diseases that were brought over by European colonizers. Over the centuries, Europeans developed resistance to many infectious diseases; however, when they reached the Western hemisphere, the diseases they brought with them triggered epidemics that devastated populations of Native Americans. This was due to a complete lack of natural resistance or immunity to many European diseases, including smallpox, measles, and influenza.
Additionally, factors such as disruption of native way of life, malnutrition from European disruption of agriculture, and the spread of diseases through dense populations and subsequent infections exacerbated the toll on Native American health. Consequently, these factors severely weakened indigenous societies, affecting their ability to resist European invaders.