| Infection |
| Viral (eg, HIV, SARS, SARS-CoV-2, influenza, measles, hepatitis) |
| Bacterial (eg, tuberculosis, typhoid fever, brucellosis) |
| Fungal (eg, histoplasmosis) |
| Parasitic (eg, malaria) |
| Congenital immunodeficiency disorders |
| Iatrogenic |
| Immunosuppressive agents (eg, glucocorticoids, antilymphocyte globulin, alemtuzumab, rituximab) |
| Chemotherapy (eg, fludarabine, cladribine), hematopoietic cell transplantation |
| Radiation therapy (eg, total body irradiation, radiation injury) |
| Systemic disease |
| Autoimmune disorders (eg, systemic lupus erythematosus, rheumatoid arthritis, Sjögren syndrome) |
| Lymphoma |
| Other malignancies |
| Sarcoidosis |
| Renal failure |
| Aplastic anemia, pancytopenia |
| Cushing's syndrome |
| Other causes |
| Alcohol abuse |
| Zinc deficiency |
| Malnutrition, stress, exercise, trauma |
| Thoracic duct leak, rupture, diversion |
| Protein-losing enteropathy |
| Idiopathic CD4+ lymphocytopenia |
