The native South Americans didn’t have Smallpox and flu and various other horrible diseases until the European explorers to the viruses there.
The reason we had them, and they Didn’t was because we had domesticate animals and had farms so we didn’t have to hunt. But living in close proximity of animals, sometimes sleeping in the same dwelling, opened us up to germs being passed from one species to another.
The native South Americans didn’t farm, and the animals they had were not domesticatable, so they never got diseases off of swine, and had no immunity when the infected Europeans came.
China is still largely a rural agricultural country, so are more likely to encounter animals than us urban city/town dwelling westerners, so it makes sense that a species jump is more likely to happen there.