When I travel abroad, which I do frequently for work, I try to run every day for an hour. It gives me energy and a great look at my new surroundings. After starting a run in Manila recently, I stopped three minutes into it. My breathing was shallow and labored. I also had nagging pain in my calves. The discomfort had been with me since I landed in the Philippines four days earlier.

So that morning I ran, walked, ran and walked for 30 minutes. I was confused, but I dismissed it as lingering jet lag and a byproduct of the intense heat. I decided to stop running for the rest of my trip, and wrote later in my running blog about how I was pushing myself perhaps too much.

Indeed, I was.

Back in Washington a few days later, I again was out of breath. But this time, I had just walked up two flights of stairs at home. Something was wrong. I opened my laptop computer, connected to Google, typed in “shortness of breath + calf pain + long flight,” and found a possible cause that turned me cold: deep vein thrombosis (DVT), or blood clotting, that could travel up to my lung and cause a pulmonary embolism, a sudden blockage in a lung artery. And that, I read, could kill me.

A half-hour later, I drove to Sibley Hospital. There, I told the admitting nurse that I believed I had DVT and cited my conditions. She checked off a box on the form for “urgent,” and 15 minutes later an ultrasound technician was looking at the veins inside my right calf.

(Arthur E. Giron For The Washington Post/Arthur E. Giron For The Washington Post)

“Oh, my god,” she said in a whisper.

“You found a clot?” I asked.

“Don’t move,” she said.

She had found two, one a large clot just below my knee. She was worried that if I moved, the clot could travel up to my lungs. An orderly wheeled me back to the emergency room, where a nurse quickly gave me anti-clotting medicines.

A doctor came a few minutes later and said that based on my story, I had almost surely had a pulmonary embolism in Manila and again in Washington. And at that moment, he said, I probably had several clots in my veins. He told me that the anti-clotting drugs would not break apart the clots — medications to do that can cause serious bleeding and generally are used only in life-or-death moments in the emergency roon. Instead, he said, I would start a regimen of two medicines— Coumadin orally and daily shots of heparin — to prevent my clots from getting bigger and further clots from developing. I was not yet out of danger, and so I would be kept in the hospital for a few days to make sure everything was okay, he said.

“You’re lucky,” he said.

Actually I felt I had been lucky twice — first in Manila and then at home.

So began my education on deep vein thrombosis. After calling my family to let them know about the diagnosis, I returned to Google and learned that 350,000 to 600,0000 Americans get DVT every year, and up to 100,000 die from it.

I found out that the biggest risk factors for DVT include surgery, immobilization, smoking, obesity, genetic tendencies to form clots (see box at right) and . . . long-haul flights. I started poking around some more and found the story of David Bloom, the NBC correspondent who died of a pulmonary embolism in Iraq in 2003. He had prolonged periods of immobility in several situations, including a long flight and time a tank during which his knees were pulled up to his chin. Bloom’s autopsy also revealed an inherited blood coagulation disorder.

DVT is frequently called “economy-class syndrome” because of the number of people who get it after sitting immobilized in cramped seats on long flights.

I started hearing from friends, including many who work in global health, a subject I’ve been writing about for the past two decades. A friend who once worked at the World Health Organization said he oncehad DVT after a long flight and now injected himself with an anti-clotting medicine 30 minutes before every long-distance trip; he said that enough people at WHO had DVT from flights that it was almost an occupational hazard.

Phyllis Kanki, an infectious-diseases professor at the Harvard School of Public Health, told me she traveled business class as much as possible in part because of the ability to elevate her legs, a way to lessen the risk of DVT. “The first thing I do once the plane takes off is to elevate my feet. It makes a big difference,” she said. “The trouble with diagnosing DVT after long trips is that so many things are going on. You get off the plane, you’re fatigued, everything feels weird, so it’s really easy not to get the right cues. If you were at home, you would pick up on them immediately.”

Several people told me about the physician John LaMontagne, deputy director of the National Institute of Allergy and Infectious Diseases, who died suddenly in 2004 from a pulmonary embolism after leaving a plane in Mexico City. I called his former boss, Anthony Fauci, to learn more about what happened.

“It was an extremely tragic situation,” said Fauci, who is also a physician. “John was sitting in the very back of a crowded plane going into Mexico City. He was seemingly doing fine, and as he was going through the passport line, he had a pulmonary embolism, and it essentially killed him right then and there.”

Fauci, one of the country’s premier AIDS scientists, said he walks a lot on flights, even if he clogs the aisles. “I have always been extremely attentive to the dangers of flying, so I spend a lot of my time walking down the aisle and getting in people’s way,” he said.

He explained how sitting in a cramped airplane seat for a long time could cause clotting: “If you are sitting down and the under part of your knee is constricted, when you bend the femoral vein, it creates more blood statis,” or stagnation, which causes blood platelets to clump up and form clots, he said.

But Victoria Day, a spokeswoman for Airlines for America, a trade association, said in an e-mail that there’s “no specific link between air travel and DVT. The risk of developing a DVT during air travel is about the same as being seated for the same period of time at a desk, in a movie theater, on a bus or in a car.”

I decided to reach out to a Web-based clearinghouse of information on DVT called ClotCare. The doctor who founded the organization, Henry Bussey, said the site receives more than 500,000 hits a month.

Bussey said that the few studies on DVT showed that long-distance flying could be a “substantial risk.” But he also said that those who had a tendency toward clotting were older people with poor circulation, women who were on hormonal medicine and people with a genetic condition that allows clots to form more easily, not necessarily people who frequently travel long distances. He suggested that my DVT could have been caused by a genetic disorder that made me predisposed to clotting.

So what does this mean for me?

I can’t say just yet. After I’m off my anti-clotting medicine this spring, my doctor will test my blood to see if I have a genetic disorder. The result matters. And what will I do when I need to fly to Manila or Addis Ababa or some other distant city? I will probably shoot myself up with an anti-clotting medicine before each flight that is longer than six hours. I’ll wear knee-high compression socks to prevent my blood from pooling in my legs. And I’ll walk up and down the aisles, drink lots of water and avoid beverages that dehydrate me.

I dodged a bullet twice. No need to do it again.

Donnelly is a freelance writer specializing in global health and the author of “A Twist of Faith: An American Christian’s Quest to Help Orphans in Africa,” to be published later this year.