Transmission electron micrograph (TEM) of particles (virions) of the simian immunodeficiency virus (SIV) surrounding an infected blood cell. The viruses appear as the small, roughly spherical objects with dark cores. SIV is responsible for an AIDS-like syndrome in monkeys, and is related to a second, more recently identified type of human AIDS virus, called human immunodeficiency virus type 2 (HIV-2). Magnification: x60,000 at 10x8 inch size.