Viral infection of brood
Sacbrood Morator aetatulas was apparently one of the first insect viruses to be discovered. When I started beekeeping in 1963, all the books had information on it, but I have rarely seen it during most of my time in beekeeping, until the early years of the 21st century, where it is now regularly seen. I don't know the reasons for the increase.
During feeding, the larvae are infected with the virus which rapidly multiplies, causing death soon after capping. The cells are uncapped by the worker bees which exposes the larvae that lay in the bottom of the cells with their heads pointing upwards. This is referred to as the "Chinese slipper" stage. The larvae change colour to yellow, then black. The larvae become liquid encased in a skin, which can be removed by the bees, either at this stage, or later when the larvae dry up.
Sacbrood may be confused with American Foul Brood (AFB) because the cappings become dark and are punctured by the bees, so be careful with your identification. It is not a serious disease, is not a killer of colonies and combs can be reused. In a bad case requeening may be advisable as there is no recommended treatment.