[ginko]

Well there was/is syphilis which is theorized to originate in the Americas and killed millions when it broke out in Europe. Apparently was more also infectious in the beginning and only turned into an STD later-on.

But Eurasia had a far larger landmass and population so there was a larger chance for nasty diseases to develop.

[Balero]

Most very dangerous transmittable diseases (like covid, TB, small pox), all moved from animals to human. Virus' in general evolve to not be particularly deadly, they can't spread if they kill the host. So the ones that do kill you, basically jump from one species to ours, and turn out to be very deadly. Bovine TB isn't a huge deal for cows, in the same way a cold isn't for us, but it'll really mess a person up.

Eurasia had much better options for domesticating a wide variety animals, so in general people lived in closer proximity to lots of animals, and generally unhygienic to boot. Which all lead to more diseases spreading over.

The America's only had a few domesticated species that deadly viruses could hop from.

[Bayart]

It's really a function of the amount of species that can be domesticated, the Americas has only few of them. Domesticated species end up living in close proximity to humans and create a perfect environment for disease.