You don't hear of American plagues decimating Europe because there basically were none. Syphilis is a new world disease, but there weren't "someone coughs, you catch the disease, it either kills you or you become immune" plagues like smallpox, measles or the bubonic plague, probably for the same reason Native Americans were pretty much neolithic while Europe was industrial: They had basically no animal husbandry.

Think of every plague you know. From the black death to Covid19. They all come from animals. To make a plague, you need to have old world style densely crowded unsanitary cities full of humans living in close proximity to animals. The animals need to have a virus, something like the cow common cold, that makes the cow mildly sick so that it sneezes on other cows to make them mildly sick, Then it needs to sneeze on a human, and the virus has to make the species switch. Patient zero of cowpox. It then spreads rapidly through the densely packed city, along the old Roman roads to other cities.

This didn't happen in the New World. They didn't have densely packed cities full of humans living alongside animals. You didn't have indians wandering through fields of sheep, goats or pigs getting covered in ungulate snot. No patient zeroes. No plagues.