There are three common causes of severe anemia: First, caused by reduced erythropoiesis: the most common anemia caused by reduced erythropoiesis is aplastic anemia, which is caused by the failure of bone marrow hematopoietic function and the inability of hematopoietic stem cell damage to produce normal red blood cells, and its anemia can often reach a severe degree. There are also pure red blood cell aplastic anemia, congenital abnormal red blood cell production anemia, and malignant clonal diseases of the hematopoietic system. Insufficient hematopoietic raw materials or due to impaired utilization, such as lack of hematopoietic raw materials, folic acid, vitamin B12. iron deficiency can also be caused. Second, hemolytic anemia: that is, caused by excessive destruction of red blood cells; third, caused by hemorrhagic anemia: either acute or chronic, such as various tumors, peptic ulcers, hemorrhoids, etc., can also cause severe anemia.