Human immunodeficiency virus (HIV) and the acquired immunodeficiency syndrome (AIDS) is a sexually transmitted disease which attacks the human immune system. It is also possible to contract AIDs from any excahnge of bodily fluids with somebody who is already infected. The disease killed 2.3 million people in 2003.
There are two schools of thought on the origin of this illness. Some people believe that humans contracted AIDs via sexual intercourse with monkeys. Monkeys have a similar illness and it is possible that it could have been passed on to humans in this way.
The other and far more sinister idea (I know right... worse than shagging monkeys) is that AIDs was bio engineered as a weapon developed by scientists working for the U.S. government by splicing together two other viruses, Visna and HTLV-1.
I wouldn't put it past the CIA to be involved in such a diabolical scheme, lest we forget MK Ultra and the heart attack gun. The cold war bought about some horrible technology and I pray that this conspiracy is one that is unfounded.