Human cells infected with Simian Immunodeficiency Virus from sooty mangabeys (SIVsm), coloured scanning electron micrograph (SEM). SIVsm is a distant cousin of HIV (human immunodeficiency virus) the cause of AIDS (acquired immune deficiency syndrome) in humans. SIVsm has transmitted to humans on several occasions where it evolved into Human Immunodeficiency Virus type 2 (HIV-2). HIV-2 tends to make people less sick than the more common HIV-1. Strikingly SIV infected sooty mangabeys almost never get AIDS despite enormous amounts of virus in their body. Studying this virus in humans and its natural hosts may help scientists understand why people get AIDS and inform development of better treatments for HIV/AIDS. Magnification: x 8000 at 10cm wide. Specimen courtesy of Greg Towers, UCL.