Thera Virus

The Thera Virus is an alien Virus that broke out on the planet Thera. The virus spread, causing the inhabitants to mutate into walking corpses and undead monstrosities. The virus infected the entire Planet, causing a Planetary Pandemic.

It's official name of the virus is "Theras-N" and humanity is at war with the creatures it creates. The war is simply known as "The Zombie War" and commenced sometime in the early 32nd millennium when the Zombie outbreaks started to occur.