I think what’s really cool is that we have traces of viral fossils in our DNA.
Viruses and us have been evolving alongside one another for a long time. As a result of this we can look into our DNA and find bits of virus in its sequences. The reason it is there is because certain viruses insert themselves into our cells during infections. These are known as retroviruses.
The presence of these “viral fossils”has been shown to influence how well a person can respond to treatments for HIV. A few years ago there was a paper which showed that if you took away a mouses ability to respond to viral infections, viruses started emerging from the mouse’s cells and increased the likelihood of the mouse getting cancer! Pretty freaky right?