The year ended on a sad note with the premature deaths of three beloved celebrities. Alan Thicke, who was most famous for playing the dad on Growing Pains, died on December 13 at age 69 after a heart attack. Pop musician George Michael passed away on Christmas Day at age 53 of heart failure. And Carrie Fisher, best known for her role as Princess Leia in Star Wars, went into cardiac arrest on December 23 and died four days later, on December 27, at age 60.

All three died from distinct, but related conditions that fall under the umbrella of heart disease, which is the leading killer of both men and women worldwide. In the United States, about 610,000 people die from heart disease annually—that’s one in every four deaths.

The term “heart attack” is often mistakenly used in place of “heart disease,” says S. Jacob Scheinerman, MD, chairman of cardiovascular and thoracic surgery at Lenox Hill Hospital in New York City. “Scientifically, the term means damage or death of the heart muscle,” he explains.

Also known as a myocardial infarction, a heart attack happens when blood flow to the heart is blocked, starving the muscle tissue of oxygen and causing damage. It’s often caused by a build-up of plaque in the arteries (called atherosclerosis).

Symptoms can be sudden, or they can build for days or weeks in advance. Thicke’s symptoms were unexpected and severe, according to TMZ: He was reportedly playing hockey with his youngest son when he started having chest pain, followed by nausea and vomiting. (Note: heart attack symptoms are often different in women than in men.)

Heart attacks are usually treated by opening the clogged artery, often with a stent or wire mesh that props the artery open. If a heart attack is treated quickly, damage to the heart muscle, which results from lack of oxygen, may be minimal. If treatment is delayed, the damage is more widespread or results in death. Sometimes death occurs quickly, even with treatment.

While heart attacks are caused by blood-flow problems, cardiac arrest results from a problem with the heart’s electrical circuitry. The electrical problem leads to an irregular heartbeat (arrhythmia), which in turn causes the heart to stop beating completely and usually without any warning.

Fisher probably had no inkling that something was about to go terribly awry.

With the heart completely shut down, Dr. Scheinerman explains, “[t]here’s not enough blood circulation, the blood pressure goes down, there’s no blood to the brain and the person faints.”

If the heart isn’t restarted within minutes, the person will die.

Efforts to revive Fisher, who collapsed on a flight from London to Los Angeles, would have involved using a defibrillator to literally shock her heart into beating again. Those efforts were apparently successful, but Fisher died in a hospital in LA on Tuesday.

While heart attacks and cardiac arrest have different causes, both can result in serious harm, even death. In some cases, a heart attack can cause cardiac arrest, although it is not the only cause.

“Cardiac arrest doesn’t have to be from a heart attack,” says Dr. Scheinerman. “It could be anything—from drugs, if you drink 18 cans of Red Bull, from arrhythmia.” (There is no suggestion that Fisher was using any drugs or alcohol.) Other forms of heart disease can also cause cardiac arrest.

Heart attacks can also cause heart failure, which is what took the life of George Michael on Christmas Day.

Most cases of heart failure are chronic and progressive, meaning they develop over years and get worse over time, and Michael supposedly had been in poor health for a while.

Heart failure does not mean the heart has failed or stopped working altogether; it means the heart can no longer pump blood efficiently. The condition has no cure, but it can be treated.

Chronic heart failure stems from many of the same problems behind heart attacks and cardiac arrest, such as coronary artery disease (which causes artery blockages), hypertension, and diabetes.

There is also acute heart failure, which can be a byproduct of chronic heart failure or of a heart attack.

All three deaths occurred at a time of the year when heart-related deaths tend to shoot up: Christmas. A recent study in the Journal of the American Heart Association speculates that this could be due to holiday stress, changed eating habits, or not getting medical care right away.