Our world is an incredible place for one simple reason: people have superpowers. With this discovery, heroes and villains rose in their ranks and their names became known across the world. People stared in awe at their abilities wondering when they'd witness the next super battle.
On the sidelines, though, the American government was busy at work. Government Produced 732, or GP-732, was a man-made virus derived from the cells of recently deceased humans. See, the government scientists were interested in creating half-alive, half-dead, easily-manipulable soldiers. This was before the virus mutated, got out of control, and escaped the supposedly sealed labs.
The virus turned people into the walking dead. Barely conscious, blood and meat-thirsty monsters that were just shells of their former beings. With delayed news of the virus, people kept their sick family members around for too long, causing the virus to rapidly spread across the world. The superheroes and villains formed a truce and used their powers and resources to try and slow down the spread of the virus while the scientists worked on a cure. However, they too soon fell to the virus and the outcome couldn't be worse.
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I've had this idea in my head for a long time, I even had some ideas planned out for it. This isn't all there is to the plot, but I don't want to reveal all my cards right now.
As for out characters, they'd obviously be survivors. Whether they, too, have powers or not is up for discussion. They could be loners, in a group/safe zone, ect.