Thrombosis is the formation of a blood clot — a thrombus — inside a blood vessel, blocking the flow of blood. Arterial thrombosis can cause heart attacks and strokes; venous thrombosis, typically in the deep veins of the leg, can send clots to the lungs as pulmonary embolism. Risk factors include immobility, surgery, smoking, obesity, certain contraceptives, and inherited clotting disorders. Diet connects in several ways: diets high in saturated fat and salt raise cardiovascular risk, while regular activity, healthy weight, and adequate hydration protect against venous clots on long journeys. Treatment uses anticoagulant and clot-dissolving medicines. It is one of the major preventable causes of death worldwide.