Death of the developing bee usually occurs either in the pre-pupal stage or just after pupation. When the vegetative rods have consumed all of the larval tissues, they turn into spores. A single diseased larva contains more than 2.5 billion spores. House bees in the colony try to remove diseased larvae and pupae and in so doing become contaminated with spores. New larvae are infected when they are fed contaminated food.
Spores are the only form of the disease that can infect new, healthy larvae.
As well, the spores can only increase in number by infecting a larva.