According to the, often tedious, timeline set out by Charles Mann in "1491", the pox hit SA around 1519 via the Caribbean. From that point onward he sees pox as the major factor in European expansion into the new world.
Oh, and mesoamerican/pre-colombian cultures were really vast and often quite advanced while Europeans didn't bathe too very much.
Please note the "often tedious" part but it's not a bad read if you bookmark a lot.