Human immunodeficiency virus (HIV) infection is an infection by viruses that progressively destroy white blood cells called lymphocytes, causing acquired immunodeficiency syndrome (AIDS) and other diseases that result from the impaired immunity.¹

The virus attacks CD4+ lymphocytes or helper T lymphocytes which help to destroy cancerous cells and invading organisms. Because HIV infection destroys CD4+ lymphocytes, it weakens the body's system for protecting itself from infection and cancer.

A healthy person has a CD4+ lymphocyte count of roughly 800 to 1,300 cells per microliter of blood. In the 1 to 2 years before recognizable AIDS develops, the CD4+ cell count usually drops more rapidly. The person's vulnerability to infection increases as the CD4+ lymphocyte count falls below 200 cells per microliter of blood. ¹