Illustration of glycoprotein spikes (GP120, pink) on the surface of a human immunodeficiency virus (HIV) binding to surface proteins on a CD4+ T-lymphocyte white blood cell, a crucial part of the body's immune system. It will enter the cell, hijack the cell's machinery to make more copies of itself, and the new virus particles will burst from the membrane of the cell, killing it. This severely weakens the immune system, causing AIDS (acquired immunodeficiency syndrome, right). The disease makes the patient susceptible to diseases that would otherwise be harmless.