There's more than one chalk brood thread on this forum . Here's part of my post from last year which I still think is relevant.

The colony that was bad with chalk brood was of medium strength and on an open mesh floor. More than 50% of the brood was infected. The queen and all the frames were destroyed and the bees were united with a nuc. For as long as I’ve kept bees in Orkney there’s always been chalk brood and some years are worse than others. I’ve tried nearly every method under the sun to try and get rid of it, apart from spraying magic potions on the bees and frames if you know what I mean.
What doesn’t help in Orkney is our cold damp weather , our amm(ish) bees which are supposed to be more susceptible to it and our closed gene pool (no imports of bees to try and keep varroa out). So as you can see the dice is well and truly loaded in the favour of chalk brood up here.
Most of the bee books class chalk brood as a minor brood disease and most of the time things will improve over the summer. But it’s also capable of making a colony unsustainable. How much should we tolerate in a hive, a few cells per frame, a few dozen cells, 25%, 50% or more? I don’t have the answer but would I be wrong to call chalk brood the elephant in the room? I agree with other members of this forum that more research needs to be done on chalk brood and I think no public money should be spent on queen imports.