The transmission of the incurable HIV from animals to humans may have come about through a related virus which affects cats and which has been around for thousands or even millions of years, according to recent US research.

The finding has implications for scientists engaged in the creation of HIV vaccines or drugs that would fight HIV.

It also sheds light on the way in which other viruses, such as H5N1 bird flu, spread to humans from the animals in which they developed.

Study author Robert Bambara, of the University of Rochester Medical Center in New York, found a previously unnoticed stretch of viral RNA in HIV, that closely mimics part of the human RNA strand, he and colleagues devised a plausible explanation for why it is still there after millions of generations.

He said that the extra RNA helps the virus propagate itself inside the human body, and that its discovery may suggest new ways to shut down the action of the virus using drugs.

The strand is believed to have come from the feline ancestor of the modern HIV virus.

While HIV is believed to have jumped to humans directly from a virus found in chimpanzees, feline immunodeficiency virus (FIV), which infects cats, is thought to be the virus from which its chimpanzee ancestor originated.

The researchers said that HIV-like 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 simian immunodeficiency virus (SIV).

Matthew Portnoy, of the National Institutes of Health, said that the study had broader implications beyond HIV research, and that it may impact responses to the current H1N1 flu pandemic.

He said there are many cases in epidemiology where a virus jumps between species and picks up DNA from each species in turn.