Complex sentence: It is estimated that upwards of 80--95 percent of the Native American population died in these epidemics within the first 100--150 years following 1492; the most affected regions in the Americas lost 100% of their indigenous populations.
Split into simpler sentences: It is estimated that upwards of 80--95 percent of the Native American population died in these epidemics within the first 100--150 years following 1492. The most affected regions in the Americas lost 100% of their indigenous populations.