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.

“Over the next couple weeks, I’m concerned of seeing kind of a tail where we start to see some of these long-term-care deaths,” DeSantis said Thursday at a forum in Tallahassee.

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.

Covid-19 is particularly dangerous for the elderly, who live in close quarters in many nursing homes and assisted-living facilities. Once an infection takes hold, it can race through a care home. One of the most memorable examples is the outbreak in a Kirkland, Wash., nursing home in February, which killed 43 people near the start of the pandemic.

While the numbers have shifted a bit, residents of nursing homes account for roughly 35 percent to 40 percent of all covid-19 deaths in the United States. At least 163,000 people in the United States have died of covid-19, according to a Washington Post analysis of data.

The problem in the early days of the pandemic was the unrecognized lethality and transmissibility of the virus. But long-term care facilities today face a threat from the sheer amount of coronavirus in the community surrounding them. Many states across the South and West that reopened early and quickly, before they controlled the virus, have been hit by a wave of new cases.

Much of the surge is blamed on younger adults who flocked to bars, restaurants, beaches and parties, where some picked up the infection. Many of them displayed no symptoms but were highly infectious to others — including the staff and visitors of nursing homes.

Like the pandemic itself, the phenomenon is both urban and rural at this point. In 26 Mississippi counties, many of them sparsely populated, deaths in long-term care centers account for more than half the total covid-19 deaths, state records show. Mississippi currently has 178 outbreaks in long-term care facilities.

In Lauderdale County, for example, 56 of the 94 deaths were in nursing homes, mainly during the early days of the pandemic, said coroner Clayton Cobler. Twelve of the county’s 13 nursing homes saw at least one death, he said.

Now, with the virus surging again, Cobler said he is wary of another surge in deaths. “We’ve gotten noncompliant,” he said, citing younger people partying, particularly at the beach.

Some states also relaxed nursing home visiting rules in the late spring as conditions improved. And the Centers for Disease Control and Prevention released a study this week showing that residents of a Maryland nursing home who received dialysis treatment outside the facility became infected in much greater numbers than those who did not receive that care.

When a team led by Grabowski examined nearly 3,000 nursing homes for the factors that led to a covid-19 outbreak, they found some circumstances had more influence than others. Facilities were more likely to have at least one case if they were in urban areas, were not part of chains and had a larger percentage of residents who were African American. The state where a nursing home is located also mattered.

But the odds of having an outbreak were not affected by having a prior infection violation, a large number of residents on Medicaid or receiving a five-star rating, the highest available, from federal reviewers. Ownership of a facility also did not appear to factor into whether an outbreak was likely.

Another recent study found that nursing homes with larger staffs were better able to limit outbreaks when they occur. Usually, this is done by separating infected and uninfected residents and stepping up other infection control measures, Grabowski said.

Mark Parkinson, president and chief executive of the American Health Care Association, said about 10 percent of the staff in the 14,000 long-term care facilities his group represents still have virtually no N95 masks. Those face-coverings filter out about 95 percent of airborne particles and are usually reserved for health-care personnel.

But, the bottom line, Parkinson said, is that widespread community transmission and a poor testing regimen are far more critical in sending the virus into long-term care facilities.

“The number one way we can reduce covid in nursing homes is if everybody in the country would wear a mask,” he said. “When people make the conscious decision not to wear a mask, they are making a decision that will kill older people.”

Some experts hope that, having learned lessons from the early days of the pandemic, nursing homes may be able to limit fatalities more effectively than they did in the first part of the pandemic.

Parkinson said the development of the treatments remdesivir and dexamethasone, which have proved effective in reducing the severity and duration of some cases, and other medical techniques may help during this new surge.

But with testing still inadequate and protective equipment for workers lacking in some places, it is still too early to know, said Vincent Mor, a professor in Brown University’s School of Public Health who studies long-term care. Community spread is still having the strongest influence.

“The exact same phenomenon that [took place] in New York, New Jersey and Seattle is working here,” Mor said.