Illustration of the various components of the human immunodeficiency virus (HIV) that could be targeted by a vaccine. HIV attacks CD4+ T-lymphocyte white blood cells, a crucial part of the body's immune system. It enters the cell, hijacks the cell's machinery to make more copies of the virus, and the new virus particles then burst from the membrane of the cell, killing it. This severely weakens the immune system, causing AIDS (acquired immunodeficiency syndrome). The disease makes the patient susceptible to diseases that would otherwise be harmless.