“What Was the First Virus?”, from the History of the Earth, researched and written by Leila Battison, narrated and edited by David Kelly, and Art by Khail Kupsky.
The film starts out by describing how soldiers brought smallpox to Rome from afar at around 100 AD, and were horribly disfigured when they survived.
Viruses are depicted as being possibly “progressive”, “regressive” (from former cells) or even originating in replicating molecules (which might happen in the lakes of Titan).
An interesting point was that some bacteria have regressed to becoming parts of mitochondria of cells or larger organisms.
Retroviruses are particularly effective at creating new genetic material to become part of higher organisms, like the placenta of mammals.
Wikipedia embed of Wuhan-Ju-1 first SARS_CoV2 virus, p.d., click for attribution.