Scientists believe they may have found a new way to fight diseases like Aids and some forms of leukaemia.
Retroviruses insert their genetic material into healthy cells
Researchers in the United States say they have identified a chemical,
which may be able to stop so-called retroviruses in their tracks.
These viruses spread by permanently inserting their genetic material into healthy cells.
Writing in the Proceedings of the National Academy of Sciences, they said the chemical may prevent this.
Most retroviruses are difficult to treat not least because they permanently change the genetic make-up of cells.
However, humans and animals can carry harmless retroviruses - viruses that have evolved over time and pose no threat to health.
A team of scientists from across the US carried out tests on sheep as part of this latest study.
They compared a retrovirus that causes lung cancer in sheep with some of the harmless retroviruses they also harbour.
They identified a harmless retrovirus that is very similar to the virus that causes lung cancer in sheep.
This virus was unable to spread and to infect healthy cells because its viral particles were incapable of escaping from the host cell in the first place.
The scientists discovered that an amino acid called tryptophan was responsible.
But they also discovered that this retrovirus, called enJS56A1, interferes with the virus that causes lung cancer in sheep.
While much more research is needed, the scientists said the discovery could lead to new treatments to fight retroviruses.
"It could have applications," Dr Massimo Palmarini of Georgia University, who led the study, told BBC News Online.
"But I would be cautious. We need to understand more about how this works. There may be subtle differences between different retroviruses. But it has potential."
Dr Dinean Pillay, an honorary consultant at University College London, said there was extensive research into retroviruses as part of efforts to tackle HIV.
"There are many human retroviruses," he told BBC News Online. "HIV is the most severe in terms of the number of cases worldwide.
"The problem with retroviruses is that they become embedded into the cell and into the human genome.
"It can last there for a long time without the cell producing the virus. But as long as that happens, then we haven't been able to beat it."