Why didn't the explorers die off from the microbes the Indians had?
I wouldn’t know. The fact is that the exchange was mostly one-way. I suppose that’s not “fair” but that doesn’t invalidate facts. It might be because Europeans had already been exposed to and immunized by most of the plagues that existed because the reservoir of disease included all of Asia and some of Africa whereas America was a “relatively” small and isolated population.
They were immune, having had the disease previously, ( measles, small pox, chicken pox, some flu viruses, mumps, etc) or were ( through genetic selection) resistant to the diseases.
It is likely that our earliest European ancestors had died from these diseases ( such as measles) in a massive way and those who lived to propagate the explorers’ generation were more hardy and more resistant and genetically passes on this trait to their offspring.
The Native American civilizations never had a Neolithic Revolution, and never domesticated any animals except the dog, which Europeans also had, if you will recall.
And, at any rate, are you forgetting syphilis, which came from the New World.
Because the AmerIndians didn't have the epidemic variety for the most part.
Most contagious diseases are carried by or come by living in close proximity to animals. Think bird flu, camel pox and so forth.
There weren't a lot of domesticated animals in the Americas. Dogs, ducks, turkeys, llamas pretty much covers it. And those type of animals tend to have fewer human transmittable diseases then pigs, chickens or horses.