Complex sentence: Unlike the Europeans, the Native Americans had no acquired immunity, and as smallpox spread into other areas and infected other tribes, it killed between 400,000--500,000 (possibly up to 1.5 million) Native Americans.
Split into simpler sentences: Unlike the Europeans, the Native Americans had no acquired immunity to the new disease. As smallpox spread into other areas and infected other tribes, it killed between 400,000--500,000 (possibly up to 1.5 million) Native Americans.