There is a lot of discussion on what exactly the virus is.

The answer is: I don't know!

However, here is where it seems to be heading:

-X'Lanthos made the virus
-it is a DNA-targeting virus designed to only affect humans
-humans have yet to be able to combat it
-some humans turn out to be "immune" or "somewhat immune" to the virus, but they still have lower reproduction rates
-virus still exists 500 years later
-Reclaimers are still susceptible to it
-virus somehow affects the main alien animal species that now control the animal faction