Transmission electron micrograph (TEM) of particles (virions) of the simian immunodeficiency virus (SIV) surrounding an infected blood cell. The viruses appear as small, roughly spherical particles with darkly-stained 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.