This scene is the result of not treating in some manner for Varroa mites. These bees are all dead, the last remains of a thriving colony of perhaps 5000 individuals in October, wiped out by this formidable pest.
Beekeepers can expect to replay this sad scene if they do not undertake a Varroa mite mitigation system.
One can see on the red circled upturned bee one or two mites in the classic feeding position under the 5th sternite segment of the underside of the abdomen.
In addition the cells above this bee contain white, mite feces that are deposited on the inside walls of the brood cells.
Methods exist for treatment, with or without chemicals, but this will be the end result if you believe that “I don’t have mites in my colony” or “I don’t see any mites so I don’t have any”.