As neither virus family is known to insert its genetic material into the host genome during replication, the discovery was all the more unexpected.

"This was a surprise for us," said Dr Anna Marie Skalka of the Fox Chase Cancer Center in Philadelphia.

"It says that the source of our genetic material is considerably wider than we thought."

"It includes our own genes and unexpected viral genes as well."

Researchers compared 5,666 viral genes from all known non-retroviral families with single-stranded RNA genomes to the genomes of 48 vertebrate species, including humans.

In doing so, they uncovered 80 separate viral sequence integrations into 19 different vertebrate species.

Interestingly, nearly all of the viral sequences come from ancient relatives of just two viral families, the Ebola/Marburgviruses and Bornaviruses, both of which cause haemorrhagic fevers and neurological disease.

"These viruses are RNA viruses," Dr Skalka said.

"They replicate their RNA and are not known to make any DNA."

"And they have no known mechanism for getting their genetic material integrated into the DNA of the host genome."

"Indeed, some of them don't even enter the nucleus when they replicate."

That the sequences, some of which may have been integrated into the genomes more than 40 million years ago, have been largely conserved over evolutionary time suggests that they give the host a selective advantage -- perhaps protecting them from future viral infection.

"In a way, one might even think of these integrations as genomic vaccinations," said Dr Skalka.