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You don't need genetic differences or Irish slaves to explain this. Might not being further north and further inland (from the warming effects of the Gulf Stream) explain it - colder weather with fewer rats to carry the infected fleas. Also the population density would have been lower, making the plague less likely to spread.

While mosquitoes may be abundant up in the northern regions...rats aren't. You find the situation in the Alps with a very small rat population.

I think the thing to take home from this article...is that there were diseases out there...hundreds, if not thousands of years ago...which might have killed you, but if you survived...it left a note in your DNA to halt AIDS infection. Perhaps our logic today might be...its not a good thing to wipe out all diease because eventually a major disease will erupt which we should have been immune to...but we aren't because we never had measles in our lifetime.

This sounds like one of those Star Trek episodes where you could change time, but recreate totally new problems. You never truely fix a problem without causing another.

The Irish slaves were not the answer. Remember, it's the Norse in Norway that had the 90% death rate. The Norse in Sweden had a lower rate. But the Saami, who had no Irish slaves at all, and most likely didn't even know where Ireland was, had the lowest death rate of all ~ something on the order of 0%-10%.

Although the Norwegian North coast is warmer than latitude alone would dictate on account of the warming currents offshore, it isn't exactly "warm". A cold climate is necessarily dry due to the greater extent of ice and snow (which are, technically speaking, "dry").

A dry climate fosters the growth of plantlife that favors rats and other rodents.

It ain't called the Norwegian Rat for nothing (VOIR ASSI: Rattus norvegicus). Then there are the Lemmings, etc. The Northern coast certainly appears to have plenty of habitat for animals which can carry nasty diseases, e.g. black death! Take a couple of tens of thousands of years occupancy in such a place and it's possible resident human populations would develop immunities to these diseases (due to the extraordinarily high death rate they would suffer from intense contact).

Note: the usual explanation for why the Black Death didn't kill Saami is that they were barely out of the stone-age and were isolated from the primary trade routes in the rest of Europe. On the other hand these guys had a long history of cultural and trade contact with the Norse just to their South. Some have hypothesized that the Saami trade in "Soma" (crystalized form of the freeze dried active ingredient in amanita muscaria, an hallucinogenic mushroom) kept them in close contact with just about every ethnic group in a broad area from Europe to China. Even Ghenghis Khan sent a delegation to visit them.