Fawcett didn't find large populations of indigenous people because an estimated 80-95% had died from smallpox and measles spread by the first generation of European colonizers between the 16th and 17th centuries.
For me, it was the literary equivalent of a smallpox vaccination: I have never, so far as I know, used the word zestful in a novel or a story. God willing, I never will.
There was the smallpox outbreak of 1776, which got so bad that Abigail was forced to get the children inoculated, a terrifying decision in an era when inoculation carried a small but significant risk of death.
They carried with them diseases like measles, influenza, and smallpox that their people had gradually acquired genetic resistance to over thousands of years of dense civilization, agriculture, and livestock domestication.