Deep vein thrombosis (DVT) is a disease that occurs when blood clots in a vein. The affected veins are usually deep in the leg muscles but can occur elsewhere. Blood clots slow down the flow of blood. The area becomes swollen, red, and painful if a blood clot moves to the lungs. Pulmonary embolism (pulmonary embolism) can lead to severe breathing problems. DVT often affects people who are not physically active, are elderly, pregnant, or have blood disorders that increase their risk of clotting.