The presence of a blood clot in a vein or artery. Although thrombosis can in principle arise in any blood vessel and in any part of the body, it occurs in the veins of the leg in the overwhelming majority of cases. A blood clot can form when the wall of the vein is damaged at some point (due to an inflammation, for example) or when the blood flow is lower than normal (during a prolonged stay in bed, for example). In both cases, blood platelets have the opportunity to attach themselves to the wall of the blood vessel and in this way form a clot. Overweight people, elderly people, and pregnant women run a slightly greater risk of suffering a thrombosis; the combination of taking the contraceptive pill and cigarette smoking also increases the risk. Thrombosis in a superficial vein in the leg is characterized by pain, redness and swelling of the blocked vein. When a clot forms in one of the deeper leg veins, a thrombotic leg results: there is pain in the calf, with swelling in the leg and shiny skin. Superficial thrombosis is cured by simple measures (bandaging, rest). The danger in deep thrombosis is that a part of the clot may detach itself and be carried to the lungs in the bloodstream (causing pulmonary embolism). Treatment of deep thrombosis is by medication to counter blood clotting. The continued growth of the clot and the formation of new clots is halted and the body itself then breaks down the existing clot.