......The AIDS epidemic was discovered June 5 1981 , when the U.S. Centers for Disease Control and Prevention reported a cluster of pneumonia in five homosexual men in Los Angeles .

Three of the earliest known instances of HIV infection are as follows :

1---- A plasma sample taken in 1959 from an adult male living in what is now the Democratic Republic of Congo.

2---- HIV found in tissue samples from a 15 year old African-American teenager who died in St. Louis in 1969 .

3---- HIV found in tissue samples from a Norwegian sailor who died around 1976 .

Two species of HIV infect humans: HIV-1 and HIV-2 . HIV-1 is more virulent and more easily transmitted. HIV-1 is the source of the majority of HIV infections throughout the world, while HIV-2 is not as easily transmitted and is largely confined to West Africa .

HIV-a lentivirus

HIV is a lentivirus, and like all viruses of this type, it attacks the immune system. Lentiviruses are in turn part of a larger group of viruses known as retroviruses. The name 'lentivirus' literally means 'slow virus' because they take such a long time to produce any adverse effects in the body. They have been found in a number of different animals, including cats, sheep, horses and cattle. However, the most interesting lentivirus in terms of the investigation into the origins of HIV is the Simian Immunodeficiency Virus (SIV) that affects monkeys.

It is now generally accepted that HIV is a descendant of a Simian Immunodeficiency Virus because certain strains of SIV's bear a very close resemblance to HIV-1 and HIV-2, the two types of HIV .

HIV-2 for example corresponds to SIVsm , a strain of the Simian Immunodeficiency Virus found in the sooty mangabey (also known as the green monkey), which is indigenous to western Africa.

The more virulent, pandemic strain of HIV, namely HIV-1, was until recently more difficult to place. Until 1999, the closest counterpart that had been identified was SIVcpz , the SIV found in chimpanzees. However, this virus still had certain significant differences from HIV.

The 'Hunter' Theory

The most commonly accepted theory is that of the 'hunter'. In this scenario, SIVcpz was transferred to humans as a result of chimps being killed and eaten or their blood getting into cuts or wounds on the hunter. Normally the hunter's body would have fought off SIV, but on a few occasions it adapted itself within its new human host and become HIV-1. The fact that there were several different early strains of HIV, each with a slightly different genetic make-up (the most common of which was HIV-1 group M), would support this theory: every time it passed from a chimpanzee to a man, it would have developed in a slightly different way within his body, and thus produced a slightly different strain.

The Contaminated Needle Theory

This is an extension of the original 'hunter' theory. In the 1950s, the use of disposable plastic syringes became commonplace around the world as a cheap, sterile way to administer medicines. However, to African healthcare professionals working on inoculation and other medical programs, the huge quantities of syringes needed would have been very costly. It is therefore likely that one single syringe would have been used to inject multiple patients without any sterilization in between. This would rapidly have transferred any viral particles (within a hunter's blood for example) from one person to another, creating huge potential for the virus to mutate and replicate in each new individual it entered, even if the SIV within the original person infected had not yet converted to HIV.

In January 2000 however, the results of a new study presented at the 7th Conference on Retroviruses and Opportunistic Infections, suggested that the first case of HIV-1 infection occurred around 1930 in West Africa . The study was carried out by Dr Bette Korber of the Los Alamos National Laboratory. The estimate of 1930 (which does have a 15 year margin of error) was based on a complicated computer model of HIV's evolution. If accurate, it means that HIV was in existence before many scenarios suggest.

About HIV-2

Until recently, the origins of the HIV-2 virus had remained relatively unexplored. HIV-2 is thought to come from the SIV in Sooty Mangabeys rather than chimpanzees, but the crossover to humans is believed to have happened in a similar way (i.e. through the butchering and consumption of monkey meat). It is far rarer, significantly less infectious and progresses more slowly to AIDS than HIV-1. As a result, it infects far fewer people, and is mainly confined to a few countries in West Africa.

Although a variety of theories exist explaining the transfer of HIV to humans, there is no widely accepted scientific consensus of any single hypothesis and the topic remains controversial.

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