Simian AIDS virus particles, coloured transmission electron micrograph (TEM). These virus particles (small, round) are bursting out of a cell (across bottom) after using the cell to replicate. They are simian immunodeficiency virus (SIV) particles, the simian (monkey) equivalent of HIV (human immunodeficiency virus) that causes AIDS (acquired immune deficiency syndrome) in humans. There are theories that HIV passed to humans from monkeys, chimpanzees or other primates. Both viruses attack the immune system, leaving the victim exposed to potentially fatal secondary infections.