The news that Hilary Clinton suffered a blood clot after a concussion in 2012 caused people to wonder how dangerous this condition can be. After all, some kinds of clots are beneficial, like the ones that form after an injury or a cut, since they stop potentially dangerous bleeding. However, there are many conditions that can cause blood clots, and some of them are harmful—or even fatal.

What is a blood clot?

Blood clots are semi-solid masses that occur when blood hardens. When a clot forms inside a vein, artery, or in your heart, it's called a thrombus. When a thrombus breaks loose and travels from one part of your body to another, it is called an embolus. The medical disorder that results is called an embolism. For example, an embolus that gets stuck in the lungs is called a pulmonary embolism.

What is causes them?

Blood clots are caused when blood pools and thickens, and can be caused by many conditions. Situations in which a blood clot is more likely to form include:

  • Immobility: Being on long-term bed rest, crossing your legs for long periods of time when sitting, or sitting for long periods of time, such as in a plane or car can lead to blood clots. Orthopedic injuries and casts can also cause them.
  • Dehydration: Not having enough water in your body can lead to clotting.
  • Hormones: Taking birth control pills or estrogen hormones can create risk, especially in women who smoke.
  • Genetic factors: Some people have a genetic or inborn error in the clotting mechanism, putting them at greater risk for forming clots.

Blood clots are also more likely in people with cancer, recent surgery or injury, liver or kidney disease, or those who use an intravenous catheter long-term.

How common are they?

The most common types of clots in veins are deep vein thrombosis (DVT) and pulmonary embolism (PE). DVT is the formation of a clot in one of the large, deep veins in your body. It usually occurs in the legs or hips, but it can crop up in other areas as well. A PE occurs when part of a deep vein clot breaks off and flows through the blood stream to the lungs. In the lungs, the blood clot can reduce or prevent the flow of blood—and that's very dangerous.

Deep vein thrombosis and pulmonary embolism are common medical problems—in fact, they're a significant cause of illness and death in the United States. According to the Centers for Disease Control, 300,000 to 600,000 people have DVT or PE, and an estimated 60,000 to 100,000 people in the United States die each year.

What are the health risks?

Of the people who have experienced a DVT, nearly one-third develop postthrombotic syndrome, a long-term condition that can cause disability. People with postthrombotic syndrome may have swelling, pain, discoloration, and scaling of the limb with the clot. And for some people, DVT may become a chronic illness. Approximately 30 percent of those who experience a DVT are at risk for having another episode. The good news is that DVT is preventable and treatable if diagnosed correctly and early.

What are the symptoms?

The symptoms depend on the location and size of the blood clot and whether the clot embolizes.

  • Clots in the lungs (pulmonary embolism): Symptoms include sudden shortness of breath, sharp chest pain, rapid heart rate, and unexplained coughing.
  • Deep vein thrombosis in the legs: Symptoms include swelling, pain, and sometimes skin redness or a rash in the affected area.

A clot in the brain (ischemic stroke) can produce symptoms like a headache, numbness, weakness or paralysis, particularly on one side of their body.

How are blood clots treated?

Blood thinners are used to treat clots by helping them dissolve over time and preventing new ones from forming. In some cases, people at high risk for blood clots or with a history of excessive clotting may be prescribed long-term anticlotting medications. "For people taking blood thinners, it's important to follow the medication directions carefully," says Dr. Richard Rainey, medical director at Regence BlueShield of Idaho. "These medications are prescribed to treat or prevent blood clots, and must be taken as directed to avoid serious side effects. In addition, regular blood tests may be necessary to monitor for side effects and make sure the dosage is correct."

How can you prevent clots?

Some blood clots can be prevented through physical activity and a healthy lifestyle. Some important tips for preventing blood clots include the following:

  • Travel wisely. Take breaks and stretch your legs when flying or driving long distances.
  • Move around. Do some heel-toe exercises when you're sitting down, and be sure to get up and walk around as frequently as possible.
  • Keep hydrated. Drink plenty of fluids, preferably water, especially when traveling.
  • Make healthy choices. Quit smoking, watch your weight, and keep cholesterol and blood pressure at healthy levels.

It's also important to be informed about your risk. Find out if you have a family history of blood clots, and be sure to share this information with your doctor.