Anemia: The condition of having less than the normal number of red blood cells or less than the normal quantity of hemoglobin in the blood. The oxygen-carrying capacity of the blood is, therefore, decreased.

Persons with anemia may feel tired and fatigue easily, appear pale, develop palpitations and become unusually short of breath. Children with chronic anemia are prone to infections and learning problems.

Anemia has four basic causes. One or more of these causes must be operating to produce anemia:

• Hemorrhage -- bleeding
• Hemolysis -- excessive destruction of red blood cells
• Underproduction of red blood cells
• Not enough normal hemoglobin

Women are more likely than men to have anemia because of the loss of blood each month through menstruation. Iron deficiency anemia is common and in adults is most often due to chronic blood loss. This can be from menstruation or from small amounts of repeated bleeding (which can be very subtle) and in children is due mainly to not enough iron in the diet. Anemia is also often due to gastrointestinal bleeding caused by medications including such very common drugs as aspirin and ibuprofen (Advil or Motrin).

There are many forms of anemia, some of them common, others rare. They include, for example:

• Aplastic anemia
• Benzene poisoning
• Fanconi anemia
• Hemolytic disease of the newborn
• Hereditary spherocytosis
• Iron deficiency anemia
• Osteopetrosis
• Pernicious anemia
• Sickle cell disease
• Thalassemia
• Myelodysplastic syndrome and a host of other bone marrow diseases.