Disease requires mass concentration of people which was less common in America. They did transfer syphilis to Europe though.

Basically, and I'm generalizing but still, all those diseases that Europeans brought over to the Americas came from domesticated European animals. On the other hand there weren't really domesticated animals in the Americas (lamas but barely). So there weren't really epidemic diseases in the Americas to infect Europeans.

This one fact basically explains the entirety of the history between these two hemispheres.

Yeah, Europeans caught syphilis from indigenous Americans, and without any existing immunity to it there was a genuinely terrifying pandemic in Europe with worse symptoms than present-day forms of the disease (and no penicillin to treat it).

But indigenous Americans caught smallpox, measles, influenza, and typhus from Europeans, so it really wasn't a fair exchange.

Europeans had more (and more exposure to) domesticated animals. Domesticated animals are a... great way to rapidly develop and pass along new and exciting pathogens to humans.

Hunting doesn't really compare in intensity and longevity of exposure to handling (and not washing one's hands) animals for grooming and shearing, and breathing in the same stinking air as a dozen livestock through the winter, and intermittently the rest of the year.

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.

It’s cause of how European agriculture worked and how they essentially helped create new pathogens.

European medieval peasants were just soooo filthy