When I started beekeeping in the late 70s there was a concentration in the East Dorset/ West Hampshire area. It was suggested that there might be a nutritional element as the soils in that area tend to be acid and gravelly. Then somebody pointed out that there was a trail of EFB cases along the main arterial routes to the orchards of Kent to which commercial beekeepers moved their hives to pollinate apples and exchange diseases (AFB by robbing, EFB by drifting).
There may of course be a genetic component. I once bought some equipment (plastic buckets, not comb or woodenware!) from a retired beekeeper, Old by name and in years, who used to take hives to Kent. One year, having brought all his hives back to a winter holding yard, EFB was found. Instant standstill order! It was some while, however, before the Inspectorate could come round to organise a massive bonfire so he was able to study the spread of the disease from hive to hive. But there was one hive that seemed immune and, even if they deliberately tried to infect it, showed no sign of EFB. He had thoughts of keeping it to breed from but the law and common sense prevailed and it was destroyed with all the rest.
My only incidence (as far as I know!) of EFB was when, on 16th July 2000, I noticed a couple of suspect cells in a recently promoted nucleus with a young queen. I had noted a month earlier they had an excellent brood pattern but poor housekeeping. I called the Bee Inspector who took a look and told me it looked to him more like Sac Brood but he took a sample anyway and sent it off to York. They confirmed that it was EFB on 21st July just as I was about to go to Gormanston for the first time. The Inspector said that he would sort them out while I was away and treated them with Aureomycin (the newly authorised treatment) fed in syrup with green dye. I thought then, and still think, that it would have been better to burn them as, within a month, they were dead anyway, and robbed out by wasps and by bees, potentially spreading the EFB.