The virus is extraterrestrial
We know very little about it, except
It was the original inhabitant of this planet
What is a virus, but a colonizing force that cannot be defeated?
Living in a cave, underground, until it mutates and attacks
AIDS, the Ebola virus, on an evolutionary scale, they are newborns
This virus walked the planet long before the dinosaurs
Your aliens, little green men arrived here millions of years ago
Those that didn't leave have been lying dormant underground
Since the last ice age, in the form of an evolved pathogen
Waiting to be reconstituted by the alien race
When it comes to colonize the planet, using us as hosts
Against this, we have no defense, nothing, but a weak vaccine
Survival is the ultimate ideology
The original inhabitant of this planet