The novel coronavirus is surging back into U.S. nursing homes, where it killed tens of thousands at the start of the pandemic and now once again threatens some of the people most vulnerable to covid-19, the disease caused by the virus.

The development is a discouraging result of widespread community transmission of the virus in many parts of the country and in hot spots where it is even less controlled. With staff — and in some cases patients and visitors — entering and leaving facilities, the community-acquired infection almost inevitably finds its way inside.

“The strongest predictor of whether or not we’ll see cases in [a particular setting] is community spread,” said David C. Grabowski, a professor of health care policy at Harvard Medical School, who studies long-term care. “We saw that in the Northeast and now, unfortunately, we’re seeing it in the Sun Belt states.”

Federal data cited by two long-term care associations this week show that the number of new cases in nursing homes bottomed out at 5,468 during the week of June 21 and has climbed steadily upward to 8,628 for the week of July 19, the last week available. That is a 58 percent increase that roughly parallels the rise in U.S. cases overall during that time.

Deaths in nursing homes from covid-19 dropped sharply to 1,324 for the week of June 28 before leveling off over the next three weeks, according to the American Health Care Association and the National Center for Assisted Living, which represent long-term care facilities. But a rise in deaths usually lags several weeks behind an increase in cases, and officials are watching for signs of increased fatalities.

On Thursday, Florida Gov. Ron DeSantis (R) spoke directly to that issue, warning that more covid-19 deaths at nursing homes and assisted-living facilities may be on the way, even as total caseloads have begun to decline. Florida has suffered a huge outbreak of coronavirus cases in recent months and has a sizable population of elderly residents.

A separate analysis of 35 states released July 21 by the Kaiser Family Foundation found that cases in long-term care facilities jumped 11 percent in the two weeks ending July 10. But in 23 hot spot states, they rose 18 percent, compared with just 4 percent in 12 states that had the virus under better control. That review included nursing homes, assisted-living facilities and other kinds of long-term care....