Remnants of ancient viral pandemics in the form of viral DNA sequences embedded in our genomes are still active in healthy people, according to new research my colleagues and I recently published.

HERVs, or human endogenous retroviruses, make up around 8% of the human genome, left behind as a result of infections that humanity’s primate ancestors suffered millions of years ago. They became part of the human genome due to how they replicate.

Like modern HIV, these ancient retroviruses had to insert their genetic material into their host’s genome to replicate. Usually this kind of viral genetic material isn’t passed down from generation to generation. But some ancient retroviruses gained the ability to infect germ cells, such as egg or sperm, that do pass their DNA down to future generations. By targeting germ cells, these retroviruses became incorporated into human ancestral genomes over the course of millions of years and may have implications for how researchers screen and test for diseases today.

Active viral genes in the human genome

Viruses insert their genomes into their hosts in the form of a provirus. There are around 30 different kinds of human endogenous retroviruses in people today, amounting to over 60,000 proviruses in the human genome. They demonstrate the long history of the many pandemics humanity has been subjected to over the course of evolution. Scientists think these viruses once widely infected the population, since they have become fixed in not only the human genome but also in chimpanzee, gorilla and other primate genomes.