Nearly 2,500 long-term care facilities in 36 states are battling coronavirus cases, according to data gathered by NBC News from state agencies, an explosive increase of 522 percent compared to a federal tally just 10 days ago.

The total dwarfs the last federal estimate on March 30 — based on “informal outreach” to state health departments — that more than 400 nursing homes had at least one case of the virus.

The full scale of the virus’ impact is even greater than NBC News’ tally, as key states including Florida did not provide data, and nursing homes across the United States are still struggling for access to testing.

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The toll of these outbreaks is growing. NBC News tallied 2,246 deaths associated with long-term care facilities, based on responses from 24 states. This, too, is an undercount; about half of all states said they could not provide data on nursing home deaths, or declined to do so. Some states said they do not track these deaths at all.

Nursing home residents are among those most likely to die from the coronavirus, given their advanced age and the prevalence of other health conditions. But the federal government does not keep a formal tally of the number of coronavirus deaths in nursing homes or the number of facilities with infections, the Centers for Disease Control and Prevention said.

Experts say more comprehensive data is critical to battling the virus and understanding why it is spreading faster in some nursing homes than others.

“It’s impossible to fight and contain this virus if we don’t know where it’s located,” said David Grabowski, a professor of health care policy at Harvard Medical School, who added that more information-gathering and transparency could help protect against future outbreaks. “You could see where it could be headed next,” he said.

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Toby Edelman, senior policy attorney at the Center for Medicare Advocacy, a nonpartisan legal advocacy organization, agreed.

“It’s critical to have accurate information about which nursing homes have residents with confirmed cases of COVID-19,” she said, referring to the disease caused by the coronavirus, “and which facilities need more staff and personal protective equipment, so that states can target additional resources where the need is greatest.”

The Centers for Medicare and Medicaid Services — the division of the federal government that oversees long-term care facilities — said states must comply with state and local reporting requirements for coronavirus cases. The agency referred questions to the CDC, which declined to comment.

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Absent federal reporting requirements, there is large variation in state efforts to gather information on coronavirus infections in nursing homes and their willingness to disclose it.

Nearly 60 percent of the deaths tallied by NBC News occurred in New York, where more than 1,300 residents of nursing homes and assisted living facilities have died, according to the state health department. In Washington state, which had the country’s first nursing home outbreak, there are 221 deaths associated with long-term care facilities. Illinois, Louisiana, New Jersey and Connecticut all reported more than 100 deaths.

The death tolls in most states include only nursing home residents. But a few states, such as Washington, include staff members.

Some states with the largest coronavirus outbreaks — including California, Michigan and Pennsylvania — did not provide the total number of deaths in long-term care facilities.

Sixteen states released the names of the nursing homes with infections. Some states have only published such details after public pressure for greater disclosure. Two Democratic senators sent a letter to federal health officials last week demanding a complete list of affected U.S. facilities.

Maryland and Ohio declined to release the names of affected facilities or the total number of nursing home deaths because of state privacy laws, state officials said, despite an outbreak that has killed 18 residents in a single Maryland nursing home and at least 40 nursing home deaths in Ohio.

Georgia provided a list naming the facilities with infections, but declined to specify the numbers of coronavirus cases and deaths in nursing homes because the figures were “too dynamic,” according to Nancy Nydam, a state health department spokeswoman.

Other states say they are actively working to bring such information to light: In Massachusetts, officials say they aim to include nursing home deaths as part of the state’s daily report on the virus, published online, as Connecticut already does. Colorado, South Carolina, Tennessee, Wisconsin and Virginia also said they were working to provide more detailed information.

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Some states, however, said they lacked the technology and resources to gather basic information on nursing home infections and deaths.

“This is not information consistently entered into the Michigan Disease Surveillance System reporting system by local health departments, and we don’t currently have the infrastructure within that system to collect the information and report it out,” said Lynn Sutfin, a spokeswoman for the Michigan Department of Health and Human Services.

Kansas, South Dakota, Alaska and Washington, D.C., did not respond to requests for data.

Even numbers provided by state health departments are likely to significantly undercount the total, given the limited access to testing and other constraints, state officials and public health experts say.

Nevada, for example, reported 20 long-term care facilities with COVID-19 infections, but said the data only reflected facilities “that proactively reported symptomatic staff and residents and have had laboratory-confirmed cases or suspect cases with laboratory testing in process,” according to a document provided by the Nevada Division of Public and Behavioral Health.

“Universal testing for COVID-19 is not available for all staff and residents,” the Nevada document added.

The same obstacles also mean that many coronavirus deaths are not being counted nationwide.

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Meanwhile, the virus has continued to rage through nursing homes across the country, many of which lack adequate equipment to protect their residents and staff.

A leading industry group said that more data would not affect nursing homes’ response to the crisis.

“We are suggesting that providers act as if COVID is already in their building, even if there are not confirmed cases,” the American Health Care Association, which represents nursing homes, said in a statement. “While more data is helpful, knowing the number of infections will not change the way our providers are reacting to prevent and contain the spread of the virus.”

In the Atlanta area, coronavirus cases are quickly rising, and eight nursing home residents have died as of Friday, according to the Fulton County Board of Health.

“At first, it was just a trickle, and it seems like it just multiplied dramatically over the last week or so,” Dr. S. Elizabeth Ford, the county’s interim district health director, said. “We need to know where the clusters are so we can direct those resources.”