As far as I know, there is really not much at all difference between populations on the first or the last, with exception of stuff like malaria resistance which probably falls under biochemistry.
And Progressive specifically mentioned that populations without exposure suffer higher mortality, and he's not talking about that. This leaves inherent, genetic immunity which is rare.
EDIT
Obviously, we are talking about much higher mortalities: if population A is genetically resistant or flat-out immune then they are going to suffer negligible deaths, so if population B suffers even something like 5% mortality it can easily be 10 times the one suffered by population A.
There is a huge difference. Lets start with the basics. For genes that help the immune system, you want lots of types with lots of diversity. This makes it much harder on the germs. So in the human immune system, we see some of the greatest diversity of all our genes, and this is responding to genetic pressure. These genes all have at least 10 major variations, and most have hundreds of variations. On some of these genes, humans have 30 million old copies. Some living humans have parts of their immune system that are closer to Great Apes than other humans. HUGE selective pressure here.
Now lets talk about isolated populations, of which the Native Americans are the best known. They left Africa with the same genetic pool as the Europeans. But then these people spent many thousands of years slowly moving across Siberia, Frozen East Asia, Beringia, and finally into the Americas. At this time due to low population densities, parasites are the cause of most disease, not infectious disease. Most/All of these parasites are lost over this slow process because of poor adaptation to cold climates (malaria, and every other parasite which has a phase outside of humans) and even lower population densities. So we get this largely parasite free population in Alaska.
Then another effect sets in. Many of these parasitic worms suppress the immune system. So does TB. Without this suppression, auto immune disease become worse. So we now have 20,000 years selective pressure for a weaker immune system. This can be see in a lot of ways. Before I mentioned how an European valley might have hundreds of different type of genes at one gene location. American Indian villages could be 50% one type of gene. American Indians often have fewer autoimmune diseases such as Type I diabetes. American Indians also have the less efficient immune response to dealing with TB.
So then the Europeans come over. The disease run rampant, and keep the population down. You can look at the Central Mexico population records to see the data. Then as we have mixing of genes and continued selection pressure, we see the immune systems slowly improve. And then the rapid rise of the population. Even low single digit % of gene mixture can dominate specific parts of our DNA if matched with heavy selection pressure.
Now I am summarizing what in fairness would take hours to cover in any reasonable amount of detail. But it is clear that these isolated population have inferior genes when faced with the modern urban diseases.