HIV retrovirus. Schematic illustration of the structure of a retrovirus, such as the HIV-1 virus that causes AIDS. At centre are RNA strands (red) making up the viral genetic material. The RNA is enclosed in a core protein shell (green). The outer envelope (yellow) consists of a lipid bi- layer, with protein spikes (orange). In the HIV virus the spikes consist of glycoprotein gp120 and it is these that recognise CD4 markers on white blood T-cells, causing T-cells to become infected. Retroviruses use their RNA to infect cells in a particular way. The HIV virus kills T-cells after infection, damaging the human immune system.