The earliest stored blood sample which has tested unequivocally HIV positive was taken from an unknown patient near what is now Kinshasa in the Democratic Republic of Congo in 1959.

AIDS was first identified as a distinct syndrome in 1981 as the consequence of a cluster of cases amongst gay men in US cities with established gay communities. A network of gay men has been traced who came from Los Angeles, New York San Francisco, Florida and Texas US cities who all appear to have been connected sexually between 1978 and 1981 and who subsequently died of AIDS. One was Gaetan Dugas, the air steward who was identified by Randy Shilts in his book And the Band Played On as the ‘patient zero’ of the US epidemic. Dugas was displaying AIDS-related symptoms in 1979, but some of his contacts became ill too very shortly afterwards so it is impossible to tell who was in fact the US ‘index case’.

Dugas had also travelled widely to Europe in his work, and apart from Arvid Noe, a Norwegian sailor who died in 1976 (see below), several other early cases of AIDS have been found in Europe, including two contrasting Germans – a soldier and an orchestral conductor – who both died in January 1979. It is possible that HIV spread from Europe to America rather than the other way round.

Retrospective analysis has also found large numbers of AIDS cases appearing in the Kagera and Rakai districts of Uganda in 1981-2. The theory is that HIV was brought to Uganda by Tanzanian soldiers who invaded in 1978 in order to overthrow Idi Amin. The circumstances of war, with violence, commercial sex and rape suddenly erupting in towns that were also key stopping points in a cross-Africa truck route, were what appear to have turned AIDS from a rare to a common disease in Africa.

What we can say is this. HIV undoubtedly first appeared in western and central Africa. Two distinct, though related immunodeficiency viruses have crossed into human beings from two different species – the less virulent HIV-2 from the Sooty Mangabey, and HIV-1, the cause of the global epidemic, from the western subspecies of chimpanzee, Pan troglodytes troglodytes (more on this below). HIV-1 appears to have jumped the ‘species barrier’ not once but three times, as evidenced by three distinct subspecies circulating in humans – type M, the main type and cause of the global epidemic, and types O and N, which are still largely restricted to central West Africa. HIV-2 also appears to have crossed over several times.

Exactly when these different varieties of SIV, the simian immunodeficiency virus, jumped the species barrier and became HIV may be forever impossible to establish, but our best guess, based on studies calculating the rate of genetic change (also seen below) is that several events could have happened between 1910 and 1950, with the maximum likelihood around 1930.

The important fact here is that HIV appears to be a very recent infection, unlike (say) tuberculosis, which has afflicted humanity for hundreds of thousands of years. Like SARS and H5N1 bird flu, HIV is an ‘emergent virus’, something new in nature (although SIV species could have crossed into humans in centuries before and never given rise to epidemics).

It is its very novelty to the human immune system that gives HIV its lethal nature. It is not in the interest of microbes to kill their hosts too quickly, and viruses tend to attenuate (weaken) over time. Most monkeys live perfectly happily with their strains of SIV and do not develop illness despite having high viral loads.

HIV may already possibly be getting less virulent: a Belgian study in 2005 (Arien) found that HIV from blood samples taken in 2002-3 was only 55% as fit as HIV taken from 1986-9 samples (though still fit enough to cause AIDS).