Oops. Turns out I was wrong about the greatest biological disaster in history. It wasn't the Black Death after all.
It was the Small Pox infestation in North America among the indigenous populations.
The Black Death killed between 65% and 80% of native Europeans. The Small Pox Infestation possibly killed upto 90% percent here the inhabitants in America.
The reason was that the native population didn't have the kind of immune system that deal with plagues. The reason it couldn't was because they didn't have them. Which was a good deal for the Europeans since the diseases only went one way and Europe didn't get wiped out by an America Pox.
The interesting question there is why? Why didn't they have plagues in America before the arrival of Europeans.
The answer to the that one comes in three parts because you need three components to get yourself a really good plague.
First you need domestic animals. Pretty much all plagues started out as diseases born by domestic animals. The thing to remember about plagues are that they are rare in nature because a germ that wipes out it's host population is like a farmer who eats his own seed corn. It would starve itself to death before it could get anywhere.
We got Chicken Pox from...duh...chickens as well as the Flu. We got Tuberculosis and Small Pox from the cow. We may have gotten the Black Death from Sheep...Although this last assumes that the Black Death was in fact a rather viscous strain of Anthrax rather than the Bubonic Plague. The Black Death was diagnosed as the later in the 1800 and was never really looked at again until recently. Regardless either way, animals were the initial carrier of what was to them a more or less harmless passenger.
The germs themselves had no idea we weren't chickens, cattle or goats and were as surprised as we were when were taken suddenly dead because of them.
Here is the other thing. It is very, very rare for a disease to jump species like that. You have to be in close proximity for a long time.
In Euraisa you had a lot more animals that were domesticated because they had a lot more good candidates for that in the first place. There are really only four boxes on the domestication checklist but you do have to hit all of them.
They have to be reasonably friendly.
They have to be easy to feed, which is to say they will eat the stuff you won't.
They have to both breed and grow quickly.
And they need a family structure that you can subvert. Dogs for instance think of us as members of the pack.
So the first thing you need for a plague are animals that can be domesticated in the first place. There only animals in the Americas that were ever domesticated by the locals were Lamas and Alpacas. Lama's do have diseases but they were kept remotely in the mountains.
The Second thing you need is cities. A village of say twenty or so that gets hit with a plague dies out immediately. And the thing about cities is that almost none of them had self-sustaining populations until the 19th century. People in cities always died out faster than then they could be replaced by breeding. Cities were kept alive by rural migrants moving in.
The last thing you needed for a good plague was domestic animals inside the cities. As I said diseases jumping species is exceptionally rare. You can have single families tending their flocks for generations safely and without incident. But you can raise the odds of transpecial contagion significantly when you put them in cities. Open air slaughter houses are a big help as are the heeps of human and animal manure everywhere.
In America the lamas never got to the cities.
And there you have it. Why Europeans were able to move into the Americas in a way that they couldn't in Africa. Basically because the human hometown had enough contact with Eurasia enough to gain some degree of plague immunity.