PHILADELPHIA — Anna Marie Bresnan, who lives at Philadelphia Protestant Home, a retirement community in Northeast Philadelphia, is 84 and has chronic obstructive pulmonary disease.

John and Kitty Stagliano, of Exton, are both 82 and have diabetes and high blood pressure.

Norma Cammisa is 93, has dementia, takes medicine for high cholesterol, and lives in a nursing home in Collingswood.

All of them caught the coronavirus. All of them survived.

Why they did so well when thousands of other people over 80 have succumbed to the new disease is a mystery that intrigues and heartens physicians and aging experts. In New Jersey, 47 percent of the more than 12,000 people who have died of coronavirus were 80 and older. As of June 5, 58 percent of Pennsylvania's 5,886 deaths were in that age group. Age, plus chronic health problems such as heart and lung disease or diabetes, greatly increases the odds that people with COVID-19 will get very sick or die.

Even in nursing homes, which are populated by frail elders who need hands-on care, a high percentage of residents who test positive for the virus have had no symptoms or mild ones. Most survive.

Joshua Uy, a Penn Medicine geriatrician who is medical director of a West Philadelphia nursing home that had the city's first coronavirus outbreak, said about a third of the 22 residents there with confirmed coronavirus were asymptomatic, a third had mild symptoms, and the remainder got very sick. Five died.

“We had a 96-year-old guy who never had a symptom,” Uy said. Some with mild symptoms have “recovered and it's like nothing ever happened to them.”

Uy couldn't predict which residents at Renaissance Healthcare and Rehabilitation Center would be fine and which would “crump,” or go into rapid respiratory failure. Patients with obesity — another big risk factor — and frailty sometimes lived. One resident with serious lung disease survived. “It's really amazing to me,” he said. “It blows my mind.”

Early in his center's two-week outbreak, he felt hopeless. “On my worst day, I was worried that they were all going to die, to be honest.” Then widespread testing revealed how many residents were asymptomatic. Some with symptoms began getting better. “When you look at the numbers,” he said, “I think most people will survive it. It just doesn't feel like it at the moment.”

Other nursing home medical directors described similar proportions of residents with mild illness and equally surprising survivors. Nina O'Connor, chief of the University of Pennsylvania Health System's palliative care program, cared for a 101-year-old coronavirus patient with no symptoms. Jim Wright, medical director of Canterbury Rehabilitation and Healthcare Center near Richmond, Va., where 136 residents tested positive and 56 died, said one 91-year-old had poor oxygenation for a long time and kept removing her oxygen mask.

“She's in our memory center now,” Wright said in wonderment. “Her favorite thing to say is, `I love you.' She says it every time.”

Jim Clancy, executive director of United Methodist Communities of Collingswood, where Cammisa lives, said a 91-year-old who was already on oxygen for advanced lung disease survived while the virus “wiped out” people who were not as sick.

“This is such a strange, random, and devastating virus. ... I don't think any two residents have been affected the same way,” he said. Asked what was different about survivors, he said: “This is the thing. There is no rhyme or reason to it.”

Wright has started analyzing the numbers at his facility and found no clear trends. He said patients there for rehabilitation, who tend to be younger and stronger than full-time nursing home residents, were more likely to survive. There were no racial differences.

“There was nothing I could put my finger on that determined your course,” he said.

Doctor have theories about why some survive and some don't. They're waiting for science.

Scientists will sort this out eventually. In the meantime, speculation focuses on differences in the immune system, genetics, and possibly medications that could alter response to the virus. One doctor suspects that hydration and even sleeping position could be important.

Coronavirus often does not announce itself loudly in the elderly, a fact that allowed it to take hold in many nursing homes before anyone knew it was there. Instead of the classic symptoms we were all initially told to look for — fever, cough, and shortness of breath — people over 80 often lose their appetites, develop diarrhea, or become confused, agitated, or more subdued. Fevers over 99 are rare.

Sabine von Preyss Friedman, medical director of 50 facilities in Seattle, including one with an early and large outbreak, has learned to look for very subtle changes. “People look at you sideways and they don't look right, you're doing a test,” she said.

Doctors said some patients never have more than mild symptoms. Wright said some can go from no symptoms to death in a few hours. Others develop what appears to be an overreaction of the immune system, or cytokine storm, a few days into the infection. In all age groups, this is a hallmark of very serious illness. Elderly people who get this sick typically do not do well, doctors said.

George Anesi, a pulmonary and critical care doctor at Penn Medicine who sees only hospitalized patients, said the virus is harder on people the older they are.