There are several possibilities for low blood white blood cells: 1. viral infections, such as influenza virus, parainfluenza virus, cytomegalovirus, herpes simplex virus. 2. serious bacterial infections, especially negative bacillary infections, such as Pseudomonas aeruginosa infection, Pseudomonas aeruginosa infection. 3. low bone marrow hematopoietic function, such as aplastic anemia. 4. malignant tumors of the blood system, such as leukemia, metastatic cancer of the bone marrow. 5. radioactive damage, such as tumor patients undergoing radiation therapy, long-term work with radiation. 6. 5.Radiation injury, such as tumor patients undergoing radiation therapy, long-term ray work. 6.Chemical substance or drug poisoning, such as lead poisoning, mercury poisoning. 7.Tumor patients undergoing chemotherapy with drugs, oral anti-thyroid drug methimazole, etc. 8.Autoimmune diseases, such as systemic lupus erythematosus, vasculitis, etc.