Ancient viruses inherited from Neanderthals have been found in modern human DNA.
Scientists are investigating possible links between the "endogenous retroviruses", which are hard-wired into DNA, and modern diseases such as AIDS and cancer.
Researchers compared DNA from Neanderthals and another group of ancient humans called Denisovans with that obtained from cancer patients.
They found evidence of Neanderthal and Denisovan viruses in the modern DNA, suggesting that they originated in a common ancestor more than half a million years ago.
Neanderthals co-existed with our ancestors in Europe for thousands of years, but belonged to a different human sub-species.
They eventually became extinct around 30,000 years ago.
Around 8% of human DNA is made up of endogenous retroviruses, or ERVs, which are DNA sequences left by viruses which pass from generation to generation.
They form part of the 90% of the genome, sometimes called "junk" DNA, that contains no instruction codes for making proteins.
Dr Gkikas Magiorkinis, from Oxford University's Department of Zoology, who co-led the research, said: "'I wouldn't write it off as 'junk' just because we don't know what it does yet.
"Under certain circumstances, two 'junk' viruses can combine to cause disease. We've seen this many times in animals already. ERVs have been shown to cause cancer when activated by bacteria in mice with weakened immune systems."
The Oxford team now plans to look for possible links between these ancient viruses, belonging to the HML2 virus family, and cancer and HIV/AIDS.
"How HIV patients respond to HML2 is related to how fast a patient will progress to AIDS, so there is clearly a connection there," said Dr Magiorkinis.
"HIV patients are also at much higher risk of developing cancer, for reasons that are poorly-understood. It is possible that some of the risk factors are genetic, and may be shared with HML2.
"They also become reactivated in cancer and HIV infection, so might prove useful as a therapy target in the future."
Co-author Dr Robert Belshaw, from the University of Plymouth, said: "Using modern DNA sequencing of 300 patients, we should be able to see how widespread these viruses are in the modern population.
"We would expect viruses with no negative effects to have spread throughout most of the modern population, as there would be no evolutionary pressure against it.
"If we find that these viruses are less common than expected, this may indicate that the viruses have been inactivated by chance or that they increase mortality, for example through increased cancer risk."
The study, published in the journal Current Biology, was funded by the Wellcome Trust and Medical Research Council.