The condition involving a decreased number of red blood cells or a low red blood cell count is anemia. More broadly, anemia means the blood has a reduced capacity to carry oxygen, often because there are too few red blood cells or too little hemoglobin.
Anemia is not one single disease. It can result from blood loss, reduced production of red blood cells, or increased destruction of those cells. Iron-deficiency anemia is especially common, because the body needs iron to make hemoglobin, the oxygen-carrying protein inside red blood cells. Other causes include vitamin deficiencies, inherited disorders, chronic kidney disease and inflammation.
Typical symptoms include tiredness, weakness, pale skin, shortness of breath, headaches and reduced exercise tolerance, although mild or slowly developing anemia may cause few noticeable symptoms. A common mix-up is treating anemia as synonymous only with iron deficiency; iron deficiency is one cause of anemia, not the definition of the condition.