HIV-1 retrovirus. Artwork showing the structure of the HIV-1 virus, the causative agent of AIDS. The outer envelope (red) consists of a lipid bi- layer, with protein spikes (turquoise). The spikes consist of glycoprotein gp120 and are used by the virus to attach itself to the CD4 molecules on the surface of T-lymphocytes (white blood cells) which then become infected. At centre are two RNA strands (red) making up the viral genetic material. The black spheres represent the enzyme known as reverse transcriptase which enables the virus to use the resources of the T-cells to make copies of itself. The RNA is enclosed in a double core protein shell (green and purple).