In normal adults, the number of leukocytes is (4.0-10.0) x 10^9/L and the number of neutrophils is (2-7) x 10^9/L. Below the normal range is a decrease. The common causes of leukocyte and neutrophil reduction are: i. Infection factors, especially when viral infections, such as influenza virus, rubella virus, etc., can cause a low number of leukocytes and neutrophils. Second, blood system diseases, the most common are bone marrow hematopoietic failure diseases, such as aplastic anemia, including chronic aplastic anemia, acute aplastic anemia. As a result of hematopoietic failure, low white blood cells and neutrophils can occur. Third, patients with malignant tumors, after radiotherapy and chemotherapy, will also have low white blood cells and neutrophils due to the damaging effects of these treatments on hematopoietic stem cells, which inhibit bone marrow hematopoiesis.