I don't think they're saying there's no genetic evidence of immunities; just not ancient ones. Most of the DNA they examined was from BEFORE 2000 BC.
What happened after that? The rise of civilization. More people living in urban environments. The Eastern conquests of Alexander the Great meant contact between European and Indian diseases. Then the great Roman-Chinese Silk Road pushed Oriental diseases into Europe, and Occidental diseases into China.
How long did it take for some diseases to evolve, and other factors to accelerate them? Without rats, there wouldn't have been widespread bubonic plague outbreaks. What caused the growth of a massive population of rats? Grain storage, more rapid and larger means of transport. How does a bunch of rats make it onto a camel caravan, for example? Why did so many people live in an area prone to swarms of rats, and how did their cultural habits lead to an excess of fleas on humans? So you see, on just one major plague outbreak in the 14th century happened because of a variety of factors.
Spread of the Black Death in Europe in the 14th Century
Thanks for your excellent Comments and the helpful map.
What struck my curiosity was that as Asians and Europeans and farmers and hunter gatherers began to meet, interact, and expand geographically, skin color changed, but immune system DNA did not.
It seems likely that the northeastern Asians who crossed the Alaskan land bridge 12-15,000 years ago would have had a very similar immune system to the rest of Eurasia.
The best estimates for the maximum death toll from the most deadly plague in recorded European history are in the 50%-60% range.
But the estimates for the New World plagues are consistently over 90%.
I have always thought the 90%-plus claim was more political correctness than medical science.
I mean, it has to be really embarrassing that about 10,000 European soldiers conquered 100 million indigenous Americans in less than a century.