Contagion/WMG

The Pandemic Virus was in reality a failed Zombie Virus
This was assumed by most of the films viewers since they saw the trailer of Gweneth Paltrow looking somewhat Zombified that this was some sort of Zombie Apocolypse film/28DaysLater style flick. However as the films story go's on this definitely wasn't the case.

However it could be therorized that the Virus of the film might have been a experimental Zombie Virus created by the Chinese military for their own brand of unkillable super soldier. Unforunatley some of the virus' samples were accidentally released and slowly spread into a few small pockets around the world.

But the Virus was incomplete and instead of creating legions of the Undead, it created just well...the Dead. The Virus itself was probably only at a 1st or 2nd stage level of infection which appeared to start the Human to Zombie Conversion but just stops short of the host actually rising, shuffling around and shouting for brains.