Anemia is a decrease in the amount of oxygen-carrying substance (hemoglobin) found in red blood cells. Anemia causes weakness, pale skin, and general tiredness (fatigue).
Anemia can be caused by blood loss or bleeding, an increase in the destruction of red blood cells, or a decrease in the production of red blood cells. Types of anemia include iron deficiency anemia, folic acid deficiency anemia, and vitamin B12 deficiency anemia, among others. Each type of anemia is treated differently.