Very low white blood cells can be seen in infections, hematologic disorders, and radiation damage, as follows: i. Infections: Viral infections are most common, such as influenza virus, parainfluenza virus, cytomegalovirus, and herpes simplex virus. In addition, severe bacterial infections, especially gram-negative bacillus infections, can also cause a significant decrease in white blood cells. Second, hematological system diseases: lead to low bone marrow hematopoietic function, such as aplastic anemia, leukemia, myeloma, etc. Third, radiation damage: after radiation therapy for tumor patients, it can lead to a significant decrease in white blood cells, and long-term engagement in radiation work can also lead to a decrease in white blood cells. Chemical substance or drug poisoning: such as oral anti-thyroid drug methimazole tablets, and lead poisoning, mercury poisoning, etc. V. Autoimmune diseases: such as systemic lupus erythematosus, etc. Clinically, the conditions that lead to extremely low white blood cells are usually most common in tumor patients after radiation therapy and in hematologic diseases.