One of the most common blood diseases worldwide is anemia, which is characterized by an abnormally low number of red blood cells or low levels of hemoglobin. One of the major symptoms of anemia is fatigue, due to the failure of the blood to carry enough oxygen to all of the tissues.

The most common type of anemia, iron-deficiency anemia, occurs because the marrow fails to produce sufficient red blood cells. When insufficient iron is available to the bone marrow, it slows down its production of hemoglobin and red blood cells.
In the United States, iron deficiency occurs most commonly due to poor nutrition.
In other areas of the world, however, the most common causes of iron-deficiency anemia are certain infections that result in gastrointestinal blood loss and the consequent chronic loss of iron.
Adding supplemental iron to the diet is often sufficient to cure iron-deficiency anemia.

Some anemias are the result of increased destruction of red blood cells, as in the case of sickle-cell anemia, a genetic disease most common in persons of African ancestry.
The red blood cells of sickle-cell patients assume an unusual crescent shape, causing them to become trapped in some blood vessels, blocking the flow of other blood cells to tissues and depriving them of oxygen.