The World Health Organization (WHO) classifies HIV infection into four stages: Stage 1 (HIV infection), the CD4 + cell count is at least 500 cells per microliter; Stage 2 (HIV infection), the CD4 + cell count is 350–499; Stage 3 (advanced HIV disease), the CD4 + cell count is 200–349; and Stage 4 (AIDS), the CD4 + cell count is less than 200 or the percentage of CD4 + cells is less than 15% of all lymphocytes. Here, CD4 is linked to HIV infectious disease.