After saying something about genetics in a different post, this is something that I have seen this summer. Chalkbrood as I have always understood it is caused by stress due to a lack of something, minerals, ventilation, poor food quality, ect. I have a hive that has periodically had chalkbrood. It still maintains strong bee numbers. Swarmed twice in three years. Having made three queenless splits from this hive, two last year and one this year, all have chalkbrood. That leads me to believe that genetics also has something to do with chalkbrood.