A great extinction event encompasses the world. In an effort to help some of the remaining population survive the governments of the world work together to release an airborne plague designed to help the populations endure the hardships and survive.
The American military neglects to inform the governments, including its own of disasters that occurred during testing. Especially that the virus will help those, still alive by enabling them to live longer with less food and water, but that it will also allow some of them to survive death itself. A different kind of life that will enable the recipient to rise, move and become some horrible parody of life. No beating heart... No vestige of the old person they used to be. Some odd caricature that rise to eat and roam the nights, and spend their days secreted away in dark places.
However, these creatures are not completely devoid of thought. There is still purpose in their rotted brains, still motivation of a kind and so they continue their effort to finish what the extinction event did not...