Anemia (anaemia) is a decrease in the total red blood cell volume in the systemic circulating blood to below normal values. The determination of the total volume of red blood cells in circulating blood is technically complex and poorly reproducible, while hemoglobin (Hb) concentration, red blood cell count and hematocrit can reflect the volume of circulating blood red blood cells in most cases, so hemoglobin concentration and/or red blood cell count below the normal value is called anemia. In coastal and plain areas, anemia can be diagnosed when the normal adult hemoglobin volume is lower than 120 g/L for men and 110 g/L for women, and when the red blood cell count is lower than 4.5×1012/L for men and 3.5×1012/L for women, and the normal value of hemoglobin for children under 12 years old is about 15% lower than that of adult men, and there is no significant difference between boys and girls. It is generally higher in areas with high altitude. According to the World Health Organization, about 3 billion people worldwide are anemic to varying degrees, and tens of millions of people die each year from various diseases caused by anemia. In China, the probability of anemia is higher than that in western countries, and among those who suffer from anemia, women are significantly higher than men, and the elderly and children are higher than the young and middle-aged. About 30%-40% of infants and children suffer from anemia, mainly due to a chain reaction caused by maternal anemia, resulting in insufficient replenishment of the blood-producing substances absorbed by the newborn from the mother. The prevalence of anemia in women is 64.4%, which is directly caused by women’s menstrual periods, the dual demand for blood-producing substances by themselves and the fetus during pregnancy, and bleeding during childbirth. Patients with anemia tend to be pale, with symptoms such as dizziness, headache, tinnitus, fainting, weakness, lethargy, lack of concentration, palpitations, shortness of breath, and also with loss of appetite, nausea, abdominal distension, and abdominal discomfort. Tongue inflammation and atrophy of the tongue papillae are mostly seen in vitamin B12 deficiency and also in iron deficiency anemia. Heterophagia is a specific manifestation of iron deficiency anemia. Some patients with anemia are sometimes accompanied by hypothermia. The clinical manifestations of anemia vary greatly depending on the severity, speed of onset, and cause of the anemia. There are various causes of anemia, which are summarized into three main categories: insufficient or reduced red blood cell production, excessive destruction of red blood cells, and blood loss. The more common ones are iron deficiency anemia, megaloblastic anemia caused by folic acid and vitamin B12 deficiency, and anemia caused by bleeding and hemolytic diseases, while anemia caused by aplastic anemia, myelodysplastic syndrome, acute and chronic leukemia is relatively rare. Therefore, once the corresponding clinical manifestations of anemia appear, it is not necessary to be overly nervous and worry about having an incurable disease. If anemia is diagnosed, it is recommended to go to the hematology department to improve the next examination and treatment under the guidance of a doctor.