A condition that develops when the blood lacks enough red blood cells. These cells are the main transporters of oxygen to organs. There are three main causes of anemia: blood loss, destruction of blood cells in the body, or abnormal production of red blood cells. Iron deficiency is the most common cause of anemia. Other disorders associated with anemia include vitamin deficiencies, sickle cell disease, cancer, and many chronic conditions (known as anemia of chronic disease).