An HIV genetic stowaway that may have come from a related cat virus could help the AIDS virus transmit and replicate in people, U.S. researchers reported on Sunday.

Their finding, which has implications for designing new drugs or a vaccine against the fatal and incurable virus, may also shed light on how other viruses, such as swine flu, spread from animals to people, experts said.

And it also may help explain how an ancient virus came to cause the devastating 25-year-long pandemic of AIDS.

Dr. Robert Bambara of the University of Rochester Medical Center in New York and colleagues found the previously unnoticed stretch of genetic material in the RNA sequence of the virus. HIV is a so-called retrovirus -- it uses RNA, instead of DNA, to function.

This little bit of genetic material closely mimics a stretch of human RNA, they reported in the journal Nature Structural and Molecular Biology.

We not only found the gene, but also a plausible explanation for why it is still there after millions of generations: its presence makes HIV dramatically better at reproducing inside of our cells, Bambara said in a statement.

This suggests new ways to shut down with drugs the ability of the virus to mass produce copies of itself.

HIV is believed to have jumped to humans from a close relative called simian immunodeficiency virus or SIV, which infects chimpanzees.

Feline immunodeficiency virus (FIV), which infects cats, is thought to be the virus from which SIV originated and therefore an ancestor of HIV, the researchers wrote.

HIV-related viruses have been identified in sheep, goats, horse, cattle and cats, but only the cat virus FIV seems to be a close relative of HIV and SIV.

The gene Bambara's team found looks very much like human tRNALys, which HIV needs to replicate itself. Like all viruses, HIV lives by infecting cells, hijacking their machinery and turning them into factories that make copies of the virus.

Determination of the origin of the tRNA-like sequence should provide valuable clues about the ancestry of HIV, the researchers wrote.

Studying this genetic sequence more may help scientists understand how viruses jump from animals to humans, added Matthew Portnoy of the National Institute of General medical Sciences, one of the National Institutes of Health.

The study has broader implications beyond HIV research, and may impact the response to the current H1N1 flu pandemic, where that virus has jumped multiple species and picked up several parts of its genome from each of the many species it has passed through, Portnoy said in a statement.

Understanding the mechanisms of these transfers enables researchers to better understand the evolution of viruses, and hopefully to better predict their 'next move' as they design vaccines and treatments, Portnoy said.

HIV now infects an estimated 33.4 million people, according to the United Nations, and has killed 25 million. H1N1 swine flu is still spreading globally and has infected tens of millions.