A day after the sudden death of long-time pop icon George Michael, many fans are struggling to understand how the fit-looking 53-year-old singer suddenly died of a heart condition.

Confirmed by his London-based publicist on Christmas Day, Michael died in his sleep of heart failure.

Heart failure, also referred to as congestive heart failure, happens when someone’s heart muscle doesn’t pump blood as well as it should.

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Some health conditions are linked to the disease, including coronary artery disease (when the arteries in the heart narrow, cutting off blood supply), high blood pressure, obesity and diabetes, according to Mayo Clinic experts.

Michael had a history of health issues. In 2011, he was hospitalized with a life-threatening bout of pneumonia. At the time, he thanked the staff at Vienna General Hospital for keeping him alive in the intensive care unit.

While the details of what led to Michael’s death have not been publicly revealed, it occurred at a time of year when heart issues are especially likely to crop up, experts say. Studies show heart-related deaths are five percent more likely around Christmas and New Year’s, perhaps due to factors like excess food, alcohol, stress and lack of exercise this time of year. These behaviors are especially risky for people with high blood pressure, a major risk factor for heart attack and stroke.

In Michael’s particular case, unless an autopsy is performed, it will be difficult to know what happened, said Dr. Chip Lavie, medical director of Cardiac Rehabilitation and Prevention and director of the Exercise Laboratories at the Ochsner Clinical School in New Orleans.

Possible underlying causes could include any number of issues: a fatal heart rhythm disturbance, a ruptured aneurysm, a pulmonary embolism, Lavie, who was not involved in Michael’s treatment, told CBS News.

During his earlier career, Michael’s drug use and taste for risky sex were splashed across the tabloids. After years of avoiding the issue in public, he eventually came to speak openly about his homosexuality and referenced it in his music.

Michael also revealed in a 2004 GQ interview that a former boyfriend had died in 1993 of an AIDS-related brain hemorrhage.

HIV-related health issues or a drug overdose can also potentially lead to heart failure, said Lavie, but again noted that he is not privvy to any of Michael’s personal health information.

Sudden death from heart failure may not be the whole story, Dr. Dan Bensimhon, medical director of the Advanced Heart Failure & Mechanical Circulatory Support Program at Cone Health System in Greensboro, North Carolina, told CBS News.

“Very few things cause healthy people to die suddenly. I suspect he may have had a serious underlying health issue that led to his acute heart issue,” said Bensimhon, who also had no involvement in Michael’s case.

“People don’t typically die suddenly from heart failure. Progressive heart failure is a slow, insidious process through which people are very symptomatic with shortness of breath, weakness and swelling,” Bensimhon explained. “That said, people with weak hearts which cause heart failure are predisposed to dangerous heart rhythms which can kill them suddenly and that is why we place implantable defibrillators (ICDs) in these patients. I suspect the announcement that he died from heart failure is not the complete story.”

It seems especially unusual for someone Michael’s age.

“The incidence of heart failure increases with increasing age so we see it mostly in people over 60,” Bensimhon said, “but we unfortunately can see it in younger people as well due to a variety of viruses and other causes.”

It can be hard for patients to recognize what’s wrong and seek treatment. “The symptoms can be insidious at the onset, including progressive fatigue, cough and shortness of breath and often people aren’t diagnosed until they have very advanced heart dysfunction and wind up in the hospital,” Bensimhon said.

There are medications and other treatments that can reduce symptoms and extend the lives of people experiencing heart failure, he added.

Exercising, eating a low-sodium diet, maintaining a healthy weight and managing stress can help prevent heart failure, experts say.