Aids stands for Acquired Immune Deficiency Syndrome. This is a broad description of a variety of symptoms displayed by someone who has been infected by the Human Immunodeficiency Virus (HIV). It usually takes about a decade for someone who is "HIV positive" to develop full-blown Aids. Once they are infected, they are infected for life.

The virus itself does not kill. HIV attacks a person's immune system, our natural defence system, fatally weakening it over time. To be more precise, HIV sabotages a specific immune cell, known as CD4 lymphocyte. This makes Aids sufferers especially vulnerable to infections. People with Aids often succumb to illnesses such as pneumonia that a healthy person would normally be expected to fight off. Without treatment, the average survival time after developing Aids is 10 months, but in individual cases it can range from two weeks up to 20 years.

Wild chimpanzees is the most likely theory. Cases of human infection were recorded in Kinshasa, the capital of the Democratic Republic of Congo, in the 1930s. It is believed that people hunting chimpanzees in neighboring Cameroon may have contracted a mutation of the Simian Immunodeficiency Virus (SIVcpz) that managed to jump the species barrier.

Earlier this year, a team of scientists from the universities of Nottingham, Montpellier and Alabama identified a natural reservoir of SIVcpz in chimpanzees in southern Cameroon. According to Paul Sharp, professor of genetics at the University of Nottingham: "When you consider that HIV probably originated more than 75 years ago, it is most unlikely that there are any viruses out there that will prove to be more closely related to this human virus.".

The relative rarity of cases and the fact that symptoms differ significantly between individuals meant the virus was not identified for another half a century. The condition first came to public attention when gay men in the United States began dying of routine illnesses in unusually large numbers. On 5 June 1981, the US Centers for Disease Control and Prevention recorded a cluster of pneumonia in five homosexual men in Los Angeles. So strong was the apparent link with homosexuals that it was at first labeled "Gay Related Immune Deficiency". But when it became clear that half of those affected were not gay, the term Aids was coined. The scientific breakthrough came in 1983 when HIV was isolated by Luc Montagnier of the Pasteur Institute in Paris, and then a year later it was confirmed by Robert Gallo of the US National Cancer Institute.

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