The best explanation for the widespread decline in Indigenous populations after the arrival of Spanish conquistadors is the introduction of diseases. European diseases such as smallpox, measles, and influenza were brought to the Americas by the Spanish explorers, and the Indigenous populations had no immunity to these diseases. The lack of immunity, combined with the lack of proper medical treatment and healthcare systems, led to devastating epidemics that decimated Indigenous communities. This demographic catastrophe greatly contributed to the decline of Indigenous populations across the Americas.
