Complex sentence: The viruses were later carried to the New World by Europeans during the time of the Spanish Conquests, but the indigenous people had no natural resistance to the viruses and thousands of them died during epidemics.
Split into simpler sentences: The viruses were later carried to the New World by Europeans during the time of the Spanish Conquests. The indigenous people of the Americas had no natural resistance to the viruses and thousands of them died during epidemics.