Dhaka, April 20 -- But the virus kept finding frail and older residents, and one culprit became clear: The workers themselves were likely spreading it as they moved between rooms and floors, outfitted with little or no protective equipment.

The nursing home, Andover Subacute and Rehabilitation Centre II in Andover, New Jersey, which has 543 beds, was chronically short of staff and masks, and over the past two years it had received poor grades from federal and state inspectors. Residents were crowded three to a room, and as the outbreak worsened, so did sanitary conditions. Spilled food littered the floors.

Workers said they hurriedly made their rounds, dispensing medicine, changing bedsheets, feeding those who could not feed themselves ...