Washington - A decades-old paraffin wax-encased block of tissue has added a curious chapter to the history of HIV, suggesting the virus that causes AIDS has been infecting people for at least 100 years.

The most pervasive and virulent global strain of HIV began spreading among human beings between 1884 and 1924, new research published Thursday in the journal Nature revealed. Until now, the virus was thought to have originated in 1930.

Researchers say urbanization in colonial Africa at the turn of the 20th century set the stage for the AIDS pandemic.

AIDS was first recognised as a disease of young, gay men in the United States in 1981, but researchers now know that it existed for decades before it was discovered or even named. HIV can remain hidden for several years after infection and before producing any symptoms.

Michael Worobey, an assistant professor of ecology and evolutionary biology at the University of Arizona, and his colleagues recovered HIV gene fragments from a 1960 wax-embedded lymph node tissue biopsy of a woman in Kinshasa in the Democratic Republic of Congo. His team had been searching for eight years for older tissue samples in Africa in hopes of finding evidence of HIV.

The other oldest known HIV genetic sequence came from a 1959 frozen blood sample of a man, also from Kinshasa, which Worobey said "was our only glimpse into the fairly deep past of this epidemic."

More virus samples that could provide opportunities for HIV sequencing, came in the late 1970s and 1980s, an era when the world knew about AIDS.

Worobey has spent many years studying the recovery of fragmented pieces of viral DNA from archival specimens and unusual sources, in order to track when HIV first jumped from chimpanzees to man.

The woman's tissue sample was preserved in the University of Kinshasa, and no one had thought of using wax-encased tissues before.

Advancements in molecular biology enabled the researchers to use the blocks literally as a time machine to get tissue samples from several decades back, and pull the DNA from HIV out of them.

They then used the woman's sample along with a database of HIV sequences to construct a range of evolutionary trees for the viral strain. The lengths of the trees' branches represented the times when the virus genetically diverged from its ancestors.

The timing and number of these genetic mutations enabled the scientists to calibrate possible rates at which the virus evolved and the epidemic grew. Then, based on this range of rates, they projected back to the period when the trees most likely took root - in this case the dawn of the 20th century.

HIV mutates rapidly. A comparison of the 1959 and 1960 virus provided additional evidence that the common ancestor of both viruses existed around 1900. It also revealed that the amount of genetic divergence between these two HIV sequences took more than 40 years to evolve.

"It's a kind of snapshot of the long history of the evolution of the virus in central Africa," said Worobey, whose research was co- sponsored by the US National Institute of Allergy and Infectious Diseases.

HIV is one of virology's biggest challenges. It is still unclear exactly how it causes AIDS, but there is no doubt that it does.

The new research shows that its estimated period of origin coincides with the rise of urban centres in west-central Africa.

"It's a really strong suggestion that before there were towns and cities, HIV - if it crossed into humans - didn't have the conditions necessary to create a chain of infections, and it would just die out," Worobey told Nature in an interview.

Once urban centres grew, the ecological change enabled HIV to invade human populations, leading the researchers to infer that man "made some changes that took a virus that basically was not capable of becoming a human pathogen and turned it into one that was."

This has implications for stemming the disease that has neither a vaccine nor a cure, and which has killed 25 million people. An estimated 33.2 million people are currently living with HIV/AIDS.

Worobey said: "If we do everything we can just to reduce the possibility of each infected host transmitting it to the next person, I think it's reasonable to talk of driving the virus to extinction even without a preventive vaccine."