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Coloured transmission electron micrograph (TEM) of HIV-1 virus particles (yellow/gold) budding from the membrane of a T-cell white blood cell (blue). Budding virus particles that have not yet separated from the cell appear as semi-circles. HIV (human immunodeficiency virus) infects helper T-cells, which are part of the body's immune system. HIV enters the cell and replicates, before destroying the cell as the new viruses emerge through its membrane. Eventually this severely weakens the immune system, causing AIDS (acquired immunodeficiency syndrome). (KEYSTONE/SCIENCE PHOTO LIBRARY/NATIONAL INSTITUTES OF HEALTH, NIAID)