Deep vein thrombosis (DVT) or deep venous thrombosis, affects about two million Americans annually, mostly in men and women over 40. DVT occurs when a blood clot develops in a vein deep in the body. It can lead to serious conditions like pulmonary embolism, which blocks blood flow when the clot travels to the lungs. Blood clots that form in the invisible "deep veins" can be an immediate threat to your life, according to the Vascular Disease Foundation. Lean more about what causes DVT, early warning signs, treatment options and the importance of DVT awareness.