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History of the disease
The search for the origins of Aids has been dogged by political controversy.
According to the latest theory, published in Nature magazine in February and widely supported by leading experts in the field, the Aids virus first passed into people from a particular sub-species of chimp in the Central African rainforest.
Human infection occurred in the first half of the century as a result of people hunting and eating the chimps, the scientists believe. This practice continues today.
The international team, led by Dr Beatrice Hahn of the University of Alabama, say genetic tests show the main human virus, HIV-1, is closely related to a virus that infects chimps but does not make them sick.
They are now studying how common the virus is in chimps in the wild, but they face problems because the sub-species in which they found the virus - the Pan troglodytes troglodytes - is endangered.
Experts say there is evidence that HIV may have transferred to humans throughout history, but only became an epidemic in the 20th century, possibly because of increased sexual promiscuity, civil unrest and movement of people to cities.
Last year, researchers said they had found the first known case of Aids - in a Bantu man who died in 1959 in the Belgian Congo, now the Democratic Republic of Congo, and home of the sub-species of chimps.
What is the treatment?
If you want to know if you have HIV, you should contact your doctor or a sexually transmitted disease clinic about a blood test. They will usually suggest counselling before you take an HIV test to make sure you are prepared for all the implications of the result, including the impact on life insurance and mortgages. Aids organisations have reported that, in some cases, just taking the test can be enough for some companies to refuse you insurance or a mortgage.
If you test positive for the virus, there are a range of treatments you may be offered. The most popular is combination therapy, a cocktail of different anti-Aids drug, including AZT. The drugs can have powerful side effects, such as anaemia, and not everyone responds well to them.
People who take the numerous drugs have to stick to a rigid regime, but they have been shown to reduce the virus and rebuild the immune system. In some cases, the virus has been reduced to undetectable levels. However, doctors say it is too early to say yet how long they will last.