HIV infection. Coloured transmission electron micrograph (TEM) of a section through human immunodeficiency virus (HIV) particles (virions, round) in infected tissue. HIV attacks CD4+ T-lymphocytes (specialised white blood cells), which are crucial in the body's immune system. It enters the cell and makes many copies of itself, which then destroy the cell as they emerge through its membrane. This severely weakens the immune system, causing AIDS (acquired immunodeficiency syndrome). Magnification: x6,000, when printed 10 cm wide.