HIV (human immunodeficiency virus) particles, coloured transmission electron micrograph (TEM). Each particle consists of an RNA (ribonucleic acid, red) genome, surrounded by a protein nucleocapsid (coat, yellow). This is enclosed in a matrix of proteins (brown) and the whole virus is enclosed in an envelope (orange). The virus enters CD4+ Tlymphocytes (white blood cells) and hijacks the host cells machinery to copy itself. The cell dies when the virus clones burst from it, weakening the immune system and causing AIDS (acquired immune deficiency syndrome).