[deleted]
hi! not discouraging more answers here, but fyi, this is a very common question here, so do start in the FAQ
If you have follow-up questions on locked posts, do ask them here; mention the relevant user's username to notify them.
By the way, the last question
If a few natives visited Europe, consider the other way around, would the same devastation had occurred to Europeans?
is off-limits here: this sub doesn't deal in hypothetical situations. You may want to x-post that one to /r/HistoryWhatIf. However, you may be interested that some of the explorers did bring people back to Europe (either as prisoners/trophies or voluntarily), where it seems the New World people generally met with the same quick fate: death by disease.
It is mutually dangerous. Diseases went in both directions, but their was a huge reason why Europeans were far less venerable compare to the natives.
The reason is the large connection that exists within the Eurasian and African Landmass. In Eurasia you have many different people and they do a good deal of traveling. Consider something like the Mongol Empire that stretched from Europe and the Middle East all the way to China. Consider the great Islamic Empires stretching from Spain, into Africa to India.
Their was constant trade and exchange of technology and disease. Their is for example a theory that traces the plague (1400ish) from the Mongolian invasion of Vietnam all the way to Europe threw the trade routes. Now I will not argue that this theory is correct, but it shows the considerable connectedness of Europe/North Africa/Middle East/Central Asia/India/China and so on. People living in these places had already built up a resistance.
All these years of connection and exchange of disease was unleashed on the natives. They in turn gave us other diseases like Syphilis but they did not give us a huge amount of new deadly diseases, as we gave them.
Two isolated people wouldn't bring so much disease. Europeans were never isolated.