my observations..it has a lot to do with luck
tried ventilation-in vain
tried good overwintering food-in vain
tried requeening-in vain
tried spraying with aple cider vinegar-in vain
tried (actually it happened) good pasture, sunny weather-in vain
tried some medicament-in vain
all my attempts have been in vain, but now...actually last week chalk brood dissapeared, just like that, plus weather conditions have been in the last twoo weeks umm i'll say damp, rainy, cloudy...
one of theoryies is that stress is one of causes and so...installing a nuc in a bigger hive would be considered as stress. but hell, i don't know.
many advise requeening like abejaruco, i must say i tried it only once, so i should not be accepting it as a rule. but if you bought this nuc from someone, contact him and tell him about it. try to get him to give you some other hive or at least a new queen.
the general problem is, lots of us newbees go and buy a nuc without knowing what to look for. i just said "give me two, how should i know which two" now i know that reading a little literature prior buying would been helpful :roll: