Human cell 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. It's not clear why, but SIVsm seems to activate infected cells much less than HIV-1 and this is linked to largely symptom free disease. Magnification: x 5500 at 10cm wide. Specimen courtesy of Greg Towers, UCL