Kinda hard when disease pathogens you have no immunity to because you didn't grow up around pigs, cattle, sheep, and horses get introduced into your population.

No one said they were more advanced than all of Eurasia. But their urban and agricultural achievements were quite impressive. The Aztecs had a city of over 250,000 people, something the Europeans could not say since the days of the Roman Empire. They also knew how to create advanced hydraulic systems and build artificial islands.

Europeans were still living in skinned tents while the Chinese had a vast civilization.

They weren't.

America has a long history of extremely complex civilisations. They didn't develop in certain areas because they had no need to. When Europeans first landed in (North)America, the technological disparity was noticeable, but hardly dominant. What killed off the Natives wasn't technology, since the technology that would give Europeans an insurmountable edge over the Natives wouldn't exist for a century or so, what killed off the Natives was disease brought by the Europeans.

As for why the Europeans had this technology, that's due to a complex series of factors including but not limited to, geology, biodiversity, geography, and having trade links with Asia and the Middle East that the Americans didn't have access to.

It's very unlikely that any Earth contains any disease that can be communicated to a species from another planet. It's very unlikely that an alien species could infect any of our species with their diseases either.