Blood Clots: Blood is supposed to clot to help repair a blood vessel that is injured. Clots or thrombi become a problem when they form inappropriately. There are a variety of illnesses and risk factors that can lead to blood clot formation such as atrial fibrillation, heart attack, strokes, transient ischemic attacks (TIAs), pulmonary embolism, deep vein thrombosis (DVT), and more. Diagnosis and treatment of blood clots depends on the cause and severity of the clot.