Blood clots like the one that Secretary of State Hillary Rodham Clinton is being treated for following her recent concussion can occur for a host of reasons. How serious a clot is depends on where it is and why it formed. A Clinton aide would not say where hers is located.
What they are
Blood pools and thickens into a clot after an injury or because of a heart problem, clogged arteries or other condition. Clots also can break off and travel to another part of the body.
Where they occur
In leg veins (called deep vein thrombosis) or in blood vessels in the neck, brain or lungs. Leg clots are a common risk after someone has been bedridden. Clots are most dangerous when they travel to the lungs, a potentially life-threatening situation, or to the brain, where they can cause a stroke.
High blood pressure, diabetes, birth control pills, pregnancy, stroke, recent surgery, prolonged sitting, circulation problems and heart problems - especially an irregular heartbeat called atrial fibrillation - raise the chances of developing a blood clot.
Sometimes a blood thinner such as warfarin (Coumadin) is prescribed to allow the clot to dissolve by itself over time and prevent new ones from forming.