We have been around this topic a few times over the years. I recall in the 70s when I was an Alberta bee inspector, I entered a yard of overwintered hives in insulated brood boxes. The owner, and old European immigant, had been ill for a year or more and so the hives had fallen into neglect.

About two thirds of the hives were obviously dead but the other third had bees coming and going very normally. 

I opened some of each and inspected the frames. 

The dead hives were loaded with AFB but as I recall the healthy hives seemed to be free of visible disease.

It was then I became a believer and promoter of selecting for AFB resistance, or more accurately, selecting agains susceptibility.

These bees were from various sources, I assume, raised in the yard, with queens from here and there. .  Thus the diversity.

In contrast, I inspected package yards in the Peace wheer the hives were uniformely breaking down with AFB.  No genetic diversity.

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