HIV. Coloured scanning electron micrograph (SEM) of HIV (human immunodeficiency virus), which causes AIDS (acquired immune deficiency syndrome). The virus consists of a RNA (ribonucleic acid) genome, surrounded by an icosahedral (20-sided) protein nucleocapsid (coat). This is enclosed in a matrix of proteins and the whole virus is enclosed in an envelope derived from the host cell's plasma membrane. The envelope has viral proteins integrated into it, including glycoprotein spikes that aid attachment to host cells. HIV infects the immune system's T-lymphocytes, ultimately killing them, leading to a very weak immune system. Magnification: x300000 when printed 10 centimetres wide.