Sacbrood disease is caused by a virus. The virus is common in hives but only causes disease in bees that are genetically susceptible. The appearance of the disease in early spring may be due to poor detection and removal of diseased larvae by nurse bees. This is caused by an increased larva to adult ratio; however, as the colony expands and develops sufficient strength, the nurse bees are able to cope with the disease.
Sacbrood disease can become apparent after bees are stressed by beekeeping management practices such as re-queening or moving hives.