(CNN) Cardiovascular disease is the No. 1 killer around the world. It killed an estimated 17.7 million people in 2015, representing 31% of all deaths globally, according to the World Health Organization .

But not all heart disease is the same. It can affect the blood vessels to the heart or brain, heart muscles and valves, and other areas of the body. Cardiovascular diseases can require long-term treatment, or they can come on suddenly and seriously.

"That's why it's very important for all of us to learn how to do cardiopulmonary resuscitation," said Dr. Nieca Goldberg , a spokeswoman for the American Heart Association.

"That's the first step when someone collapses to help save their life," added Goldberg, who is also medical director of the Joan H. Tisch Center for Women's Health at the NYU Langone Medical Center in NYC.

What is heart failure?

The American Heart Association describes heart failure as a condition that occurs when this important organ, essentially a pump, cannot effectively push blood out through the arteries and circulatory system to the body's other organs and tissues.

Congestive heart failure, a worsening of this general condition, means blood flow from the heart through the arteries has slowed while blood returning to the heart through the veins has begun to back up and combined they cause congestion -- a blood traffic jam -- in the body's tissues.

The result is edema, or swelling, usually in the legs and ankles, though edema can happen anywhere in the body. Heart failure also impairs the kidneys' ability to dispose of water and sodium, causing even more swelling. When pulmonary edema happens, fluid collects in the lungs and interferes with breathing.

Conditions that can lead to heart failure include high blood pressure, diabetes, obesity and coronary artery disease: when plaque builds up in the walls of arteries causing them to narrow and increasing the difficulty of pumping blood.

Heart failure, then, is a medical condition that needs to be treated to prevent a life-threatening heart attack.

What is a heart attack?

"A heart attack is a circulation problem," Goldberg said. When circulation is blocked or cut off in some way and blood is no longer supplied to the heart muscle, this can damage that muscle, she explained. Though it's commonly described as a heart attack, doctors refer to this condition as "myocardial infarction."

Blockages causing heart attacks are mostly caused by a buildup of plaque in the arteries. Plaque forms when cholesterol combines with fat, calcium and other substances in the blood.

Combined, these elements harden into plaque, which can then rupture, causing a blood clot to form. Large clots can completely block the flow of blood through an artery.

"People who are at risk for heart attacks are people who have a family history of heart attack, having high cholesterol, high blood pressure, diabetes, lack of exercise, cigarette smoking -- the major risk factors we always discuss," Goldberg said.

Another less frequent cause of a heart attack is a spasm caused by tobacco or possibly illicit drugs, such as cocaine, which disables the heart muscle, according to the American Heart Association. A tear in the artery, though rare, can also result in a heart attack.

The association says heart attacks can be fatal, but they do not automatically lead to death. The group advises that immediate emergency medical help can often prevent a heart attack.

"And if you think someone is having an heart attack, call 911. Don't wait," Goldberg said, explaining that the reason it's important to take an ambulance to the hospital instead of, say, hitching a ride with a family member or friend is that the ambulance is equipped to treat cardiac arrest on the way to the emergency room.

What is sudden cardiac arrest?

Though a heart attack occurs when circulation of blood is blocked, cardiac arrest is the result of electrical disturbances that cause the heart to suddenly stop beating.

"Sudden cardiac death is an electrical problem, where your heart's rhythm is rapid and irregular and your heart can't pump effectively, so you suddenly collapse," Goldberg said.

As you might expect, a sudden, unexpected loss of heart function results in an equally sudden loss of breathing and consciousness.

Survival is possible after sudden cardiac arrest, with treatment. Once again, CPR, a defibrillator or chest compressions could save someone's life until emergency personnel arrive.

One cause of sudden cardiac arrest is a heart attack.

"Sometimes, people who are having heart attack have a complication of sudden cardiac death if they don't get to the hospital soon enough," Goldberg said. Yet most heart attacks do not lead to sudden arrest, according to the American Heart Association.

Goldberg added that another risk factor for sudden cardiac death is a genetic predisposition to heart rhythm problems. In families in which people are known to die suddenly, members are screened and closely monitored, Goldberg said.

Symptoms and numbers

The most common warning signs of a heart attack are discomfort (sometimes pain) in the chest; lightheadedness, nausea or vomiting; pain in the jaw, neck or back; discomfort in the arm or shoulder; and shortness of breath. Some of these may occur more often among women and others more often among men.

By comparison, sudden cardiac arrest strikes without warning : A person collapses and has no pulse, no consciousness and no breathing.

Overall, heart attacks are more common than cardiac arrest in the United States.

During 2014, for example, the American Heart Association calculated about 565,500 sudden cardiac arrests. By comparison, nearly 750,000 Americans have a heart attack each year, according to the association.

Worldwide, heart disease is the leading cause of death , accounting for more than 17.3 million deaths each year, a number that the American Heart Association expects to grow to more than 23.6 million by 2030. "Heart disease" includes all forms of possible heart troubles, such as heart failure, heart attack, sudden cardiac arrest, heart defects at birth, arrhythmia and cardiomyopathy (an enlarged heart muscle usually caused by genetics), high blood pressure and high cholesterol.

Though any heart problem can ultimately lead to death, the most immediately life-threatening are heart attack and sudden cardiac arrest.

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"I think it's really important for us to focus on preventing people from having heart attack through lifestyle changes," Goldberg said, suggesting not only good diet and physical activity but also the need to get routine checkups and, if necessary, treat any blood pressure or cholesterol problems.

"Interestingly enough, our rate of heart attacks in men and women have decreased over the last 10 years," she said. "I think it will take time with the recent increase in all these risk factors to see if we are going to see a bump, but I can't predict that." Though hopeful, she sighed, adding, "That's a concern of mine."