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.
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).
Well there is no questions the rats could survive in the coolder climate but the fleas probably can't.
An example is in NY with the Deer tick, In southern NY (Catskills, Teconic, etc.) the woods are full of them however further North in the Adirondacks where it's really cold there is none or few even though there is just as many if not more deer. I've hiked with my dog in the Adirondacks 1000s of times and not once did either of us ever get a tick or flea. So even if the warm blooded rats could thrive in a cold climate that doesn't mean their cold blooded fleas could.