Budding HIV particles. Coloured scanning electron micrograph (SEM) of HIV particles (blue) budding from the membrane of the host cell (orange). HIV (human immunodeficiency virus) attacks CD4+ T-lymphocytes, 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.