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AIDS virus particles (yellow/pink) budding from the surface of a T4 lymphocyte cell, coloured transmission electron micrograph (TEM). AIDS (acquired immune deficiency syndrome) is a disease caused by the human immunodeficiency virus (HIV). HIV infects T4 cells, and uses their own cellular machinery to make more copies of HIV. These copies then burst out of the cell, killing it and infecting others. T4 lymphocytes are white blood cells, and are an important part of the immune system. AIDS severely damages the immune system, making normally harmless infections life- threatening. There is no cure. Magnification: x71,500 when printed 10 centimetres wide. (KEYSTONE/SCIENCE PHOTO LIBRARY/DR STEVE PATTERSON)