Patrice i would like to clarify the source material never ventured into to origin of the virus. This was in an interview with the author (who btw was jaded in the direction of the show) and doesn't necessarily preclude from the reasons listed in this article from taking a simple virus and turning it into a weapon of mass casualty.

According to the writer of the original source material (the graphic novels) definitively said it was caused by a virus introduced by an asteroid that was in effect an alien disease.

I can tread water with just my feet and keep myself at water line at or above belly button. If i can do that imagine a creature with a half-mile long tail and giant flipper feet can do.