A good science fiction story should have some basis in science fact. Click here to read the prolog

A few years ago, scientists in Antarctica discovered a living organism believed to be about two hundred and fifty million years old.
By comparison, the virus in this story (an ancient rabies like virus) is a mere sixty five million years old.

Did an asteroid bring about the end of the dinosaur era? Possibly. We do know that a very large meteor struck an area now called the Yucatan about sixty five million years ago. We also know that a mass extinction occurred somewhere in that time frame, beause we have carbon dated a lot of the fossil record. Seems like an open and shut case, doesn’t it? On the other hand, carbon dating doesn’t give us really precise results. The half-life of carbon 14 is 5568 (±30) years. Using the assumed extinction timeline(65 million years ago), simple arithmetic yields plus or minus 350,000 years, giving us a window of 700,000 years during which the mass extinction must have occurred. Just a moment on the cosmic clock, but a long time to wait for season tickets.
One argument that many physicists have trouble accepting is the claim that the earth was shrouded in an enormous dust cloud for thousands of years, but some of you may have noticed that if you throw something into the air it doesn’t normally take a thousand years for it to fall back to the ground. Obviously the impact of a large meteor would throw a huge quantity of earth high into the atmosphere, but a thousand years is a long time, and the assumption here is that gravity was functional at the time. So, was it an asteroid? Maybe, but then again, maybe it wasn’t. One good thing about the meteor hypothesis: it left behind an easily identifiable footprint. Something to sink our teeth into, so to speak. But if it wasn’t a meteor, what was it? Well, there are a number of possibilities:

Extinction hypotheses:

A sudden pole shift:We know that our magnetic poles move about from time to time. If for some reason that happened very quickly it could have altered local climates faster than existing species could adapt.
Solar flares and radiation:Supernova of a (relatively) nearby star could have destroyed the ozone layer and incinerated the planet. Recent evidence suggests that this is a very real possibility, that the ancient apocalypse came from above and beyond.
Evolving mammals:This was the working theory when I was in grade school, and it remains a viable candidate. Evolving mammals and their taste for reptile eggs might have pushed the dinosaurs past the tipping point.
Disease:Smallpox epidemic, influenza pandemic, SARS, AIDS, plague, H5N1 avian flu... global epidemics are really rather common. What might have happened across thousands of millennia? An apocalyptic virus is certainly not unthinkable. In fact, the GAIA hypothesis claims the Earth needs to do just that from time to time

The novel Full Circle by Michael Boyle suggests that the dinosaurs were wiped out in the mother of all pandemics.
Unfortunately for us, the virus that caused it survived, frozen in the bones of a long dead beast, buried deep in the arctic ice.

Before you ask:
Yes, I know that the reports of the bacterium in Antactica turned out to be inaccurate.
Yes, I know that carbon dating is not the method we use to date fossils. However, it is the one that most people are familiar with, and the actual methods in use have about a 1% margin of error, which is not at variance with the thesis presented above.