There may be apiary effects that have not been fully appreciated. For example, everybody knows about drifting, but that basically causes some hives to get progressively stronger. 

There may be a lot a pilfering going on, where bees sneak into other less well protected hives to steal honey, thereby spreading pathogens in both directions.

This may be far more pronounced in larger apiaries, but so far as I know, it has never been seriously studied.

PLB

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