Most everyone knows that heart disease kills. Clots in the arteries block blood flow and lead to strokes and heart attacks.

But what isn't so well known is that deadly clots can also come from the veins.

If you're a frequent flier, you may well have heard of deep vein thrombosis, or DVT. That's because sitting for long periods of time can cause clots in the legs that could end up in your lungs.

DVT made headlines following the shocking death of NBC journalist David Bloom. His wife, Melanie, made a public service announcement to promote awareness of DVT, which became a deadly pulmonary embolism.

"When my husband David died from a DVT, he was covering the war in Iraq. I hadn't even heard of this condition," she said.

A Cardiologist's Experience

Cardiologist Seth Baum knows lots about DVT. Naturally, he wants the public to understand why they need to know about this.

"Some statistics suggest that as many as 650,000 people will die every year in the United States from pulmonary embolism, making it the third largest cause of death," he explained.

The grim statistic follows heart disease and cancer. Baum found out from personal experience about the dangers of DVT.

Fellow physician Dr. Phillip Snider explained that when a blood clot forms in a vein in the leg, it may stay there or it may go through valves.

When that happens, the clot "can travel up toward the heart and then go out directly to the lungs. It can travel to either lung and block it there," Snider said.

In 2007, Baum wasn't flying. He was merely planting trees on a hot Florida day when he noticed a chest pain.

"It was kind of a weird pain in that it hurt when I touched it and it felt like I had fractured a rib or torn a muscle," Baum recalled.

He knew from medical school this symptom was not usually an indicator of DVT. Unfortunately he waited two days to see a specialist.

"His words to me after he saw the results of my test were 'it's a miracle that I'm alive,'" Baum said. He had had many clots in both lungs, some of which had caused infarction, or killed, a portion of his left lung.

Baum underwent surgery to get a vein filter to prevent further clots from traveling to the lungs. He also began taking a blood thinner.

Genetic Factors

So how this could happen to a health-conscious cardiologist? One test found a genetic variation conducive to developing clots in the legs.

More than 10 million people in the U.S. are believed to carry the genetic variation called "Factor 5 Leiden." Not all physicians and very few patients even know about this threat or that there's a test for it.

There are additional genetic factors that increase clotting. So checking with family members about any of their clotting incidents can often reveal your own potential risk.

In addition to long flights, other situations, such as cancer or pregnancy, require vigilance. Snider suggests simple steps.

Exercise and Drink Water

"What you try to do is stay active, physically active, to a minimum to get up and walk around at least two or three times a day for a minute or two."

More exercise can provide even better protection. Baum's regular exercise regimen probably saved his life by strengthening his circulatory system against those killer clots.

Surgical patients also face a clot risk during recovery. Doctors put special socks on patients to keep the blood from pooling and then clotting.

One simple way to fight DVT is to drink plenty of water. That thins the blood and stops clots from forming. So if you have a water cooler at work, take advantage of it.

Baum said dealing with DVT is much different than battling cardiovascular disease, which involves so much more than a simple message. Fighting cardiovascular disease involves a total change in lifestyle, lowering cholesterol, along with a whole range of preventive strategies.

Baum said learning about how common DVT is, about its symptoms in the legs and chest, and of simple ways to combat it can make a difference. He focuses on the bottom line.

"We could very easily save literally hundreds of thousands of people a year, pretty impressive, just by getting the word out."