One universal antiviral to rule them all?

1) Viruses don't adapt instantly nor perfectly - that's why viruses can be animal-specific. Influenza (or recently SARS-CoV) are famous because they are malleable enough to adapt to new hosts, human or animal, within a few months or years, but not all viruses have this ability.

2) To further illustrate, some viruses have been nearly eliminated with a single vaccine. Polio didn't manage to adapt before going almost extinct. And a good reason why is:

3) Viruses can only evolve inside contaminated hosts. If you find a cure that stops quickly the virus from multiplying and contaminating, you are also curtailing its ability to adapt. A contaminated host is a giant casino machine, allowing the virus to mutate until it hits a new evolutionary step. A strong enough vaccine or treatment is like throwing out the virus before it has time to play much.

Two viruses have been entirely eliminated in the wild, one (Smallpox) still exists in government research facilities the other (Rinderpest) I believe is just gone because it wasn't useful as a direct weapon (humans aren't affected) and nobody actually wants Rinderpest, it was just killing cattle and while poor farmers need their cattle or they'll starve the rich want to drink milk and eat steak so they weren't keen on this virus either and helped fund its eradication.

> A contaminated host is a giant casino machine, allowing the virus to mutate until it hits a new evolutionary step.

And even worse, some viruses can swap genes if a host has multiple infections at a given time. Bats in particular are known as "hot pots".