When the coronavirus outbreak hit one of the largest and most troubled nursing homes in the Northeast, coughing and feverish residents were segregated into a wing known as South 2. The sick quickly filled the beds there, so another wing, West 3, was also turned into a quarantine ward.

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 Center II in Andover, N.J., which has 543 beds, was chronically short of staff and masks, and over the last 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 and doing other tasks that brought them into close contact with residents.