Humans and dogs may have exchanged genetic material over the millennia via viruses, scientists conjecture.

Retroviruses — the most infamous example of which is likely HIV, the virus that causes AIDS — have the ability to incorporate their genetic material into that of their hosts. In this manner, these hitchhikers can reproduce when their hosts do.

All mammals and most vertebrates, or creatures with backbones, apparently possess these "endogenous retroviruses" in their genomes. In fact, nearly 1 percent of the human genome consists of these unwelcome guests. Mice and opossums are even more greatly compromised, with these viruses making up about 2 percent of their genomes. [Deadly Diseases that Hopped Across Species]

To get a broader picture of how deeply retroviruses have invaded genomes, scientists in Sweden analyzed the first sequenced carnivore genome, that of a female dog of the boxer breed.

The researchers discovered that endogenous retroviruses only seem to make up 0.15 percent of the dog genome, six times less than humans. Dogs may have better mechanisms to protect their genomes against retroviruses, or their genomes may house unknown types ofretroviruses that current techniques can't yet detect, the researchers say.

Intriguingly, the scientists discovered a novel group of retroviral material in dogs that is highly similar to endogenous retroviruses seen in humans. They belong to a type of virus known as gammaretroviruses, the most frequent type found in mammals to date.

This specific group of retroviruses seems to have invaded the dog genome relatively recently. This suggests that dogs and humans may have passed these germs to each other due to close interactions during our millennia of history together, a phenomenon known as "lateral transmission." It remains uncertain how such transmission might have occurred — perhaps from wet doggie kisses, for instance.

"We need to stress that we can only say 'potential for possible lateral transmission between dog and human,'" said researcher Göran Andersson, a molecular geneticist at the Swedish University of Agricultural Sciences.

To shed light on if, when and how much this happened, "DNA from more dogs will be analyzed," Andersson told LiveScience.

Such research might not only discover evidence for such lateral transmission, but also could reveal how dogs might be protecting themselves against retroviruses. Such knowledge might help lead to therapies against retroviruses, including perhaps HIV, Andersson said.

The scientists detailed their findings online May 12 in the journal PLoS ONE.

Follow LiveScience for the latest in science news and discoveries on Twitter @livescience and on Facebook.