Ascosphaera apis is a fungal disease that infests the gut of the larva. The fungus will compete with the larva for food, ultimately causing it to starve. The fungus will then go on to consume the rest of the larva's body, causing it to appear white and 'chalky'.
Chalkbrood is most commonly visible during wet springs.
Hives with Chalkbrood can generally be recovered by increasing the ventilation through the hive.