The job of DNA is to make copies of proteins which are then used in the body. Haemoglobin, collagen, antibodies, keratin (which makes hair and nails)… these are all proteins encoded in the DNA.
Retroviruses are viruses that re-write the DNA of organisms, commandeering that system in order to make more copies of themselves. So instead of making haemoglobin proteins and sending them to your blood stream, the DNA on your cells will make more viruses and send them to your bloodstream.
Most of the time, the DNA that retroviruses re-write will die with the carrier… but very rarely, a retrovirus will re-write the DNA in a sperm or an ovum cell, and the carrier will pass the change to its offspring (this is not the same as passing the virus itself through the placenta, the virus is permanently written into the DNA.
Every organism has literally tens of thousands or even hundreds of thousands of these remnants of ancient viral infections which have been deactivated/neutralised. They are like “fossils” of ancient viruses. They are called endogenous retroviruses (ERV) and they comprise 7.9% of our entire genetic makeup.
Yes, dead viruses comprise almost 8% of our DNA!!
The cool thing about ERVs is that they are unique. The chances that a virus infects you and re-writes your DNA in exactly the same place as another person or another animal are so minuscule that it is for all practical purposes, completely and utterly impossible. So ERVs can be used to determine the inter-relatedness of creatures. It’s another way of making a phylogenic tree, and lo-and-behold, the phylogenic tree made using ERVs exactly matches all the other trees, surprise, surprise!
It shows that all humans share all our types of ERVs the last retrovirus having infected our common ancestor 150,000 years ago…
The creatures we share the most ERVs with are… surprise, surprise, chimpanzees!!… Next, gorillas… next, orangutans… next, Old World monkeys… and so on and so forth…