CD4 refers to CD4-T lymphocytes, or CD4 cells for short. Common clinical causes of low CD4 cells include AIDS, radiotherapy, chemotherapy, and pathogenic microbial infections. 1. AIDS: Human immunodeficiency virus mainly attacks CD4-positive T-lymphocytes, which gradually decrease in number after being destroyed and may lead to low CD4 cells. 2. Radiotherapy and chemotherapy: patients with malignant tumors need to receive treatments such as radiotherapy and chemotherapy. Receiving these treatments will not only kill tumor cells, but also kill lymphocytes, which may also lead to low CD4 cells. 3. Pathogenic microorganisms infection: cytomegalovirus, hepatitis virus, measles virus and other pathogenic microorganisms infection may violate the human body’s immune system, resulting in low CD4 cells. There may be other causes of low CD4 cells, it is recommended to go to the hospital as soon as possible, and cooperate with the doctor to complete the relevant examinations to clarify the cause of the disease.