HIV virus. Illustration of a single human immuno- deficiency virus (HIV), the cause of AIDS (acquired immune deficiency syndrome). The surface lipid bilayer (orange) is shown which is studded with GP120 glycoproteins (green). It is these glycoproteins that bind to target human white blood cells (T cells). The virus attaches to receptors on the T cell membrane and injects its RNA (ribonucleic acid) from its core (unseen) into the cell. This forces the T cell to reproduce HIV which bud from the cell, eventually killing it. Decline in T cell numbers severely compromises the immune system, exposing the patient to other infections, such as pneumonia.