Leukocytes below 3 x 10^9/L are not normal, a condition known as leukopenia, which can be seen in the following situations: i. Viral infections, such as the common viral cold or influenza, chickenpox or herpes zoster, and viral hepatitis. Second, the elderly frail as well as immunocompromised patients, the emergence of serious bacterial infections, there will also be a decline in the total number of white blood cells, such as typhoid fever, paratyphoid fever bacillus infection. Third, when the hematopoietic system of the body is abnormal, there will be a progressive decline in white blood cells, such as aplastic anemia, pernicious histiocytosis, which needs to improve the bone marrow aspiration examination. Fourth, drug damage, such as long-term oral painkillers, methimazole and so on. Fifth, radioactive damage, such as malignant tumor patients after radiotherapy, there will be a significant decrease in white blood cells. Sixth, various causes of splenomegaly, hypersplenism, such as cirrhosis, portal hypertension.