HIV particle, computer artwork. HIV (human immunodeficiency virus) causes AIDS (acquired immune deficiency syndrome). The outer layer of the virus particle is a membrane envelope (green and purple) made from the membrane of the host cell in which it developed. The membrane encloses an icosahedral (20-sided) matrix layer, which surrounds the inner ribonucleic acid (RNA) genome (not seen). The red triangle on show highlights just one of these sides. Surface knobs (green) allow the virus particle to attach to host cells. HIV attacks lymphocyte white blood cells, which play an important role in the immune system. This severely impairs the immune system, allowing normally harmless infections to become life threatening.