Every single human in the world is infected. It takes only the loss of control of your limbs for the parasite to take effect. It was in the shots that they gave the children.
The children started getting sick. Massive internal hemorrhaging, vomiting blood and begging for their parents. We had to quarentine them. They started doing experiments on the corpses of the kids. Attempting to fix their fuck up.
Go figure, the Pharmaceutical companies fucked us once more. They knew there was going to be an uprising, so they decided to make an end game.
The bacteria, a strain of Bacterial Menengitis, infected the spinal fluid of the children, and then used the abundance of stem cells to adapt, to become respiratory, reproductive, or other parts of organs. That was 3 generations ago. Every single person that is alive today has the virus, and the trigger has been set off.
The corpses reanimate after a 2 hour incubation period, in which the bacteria cells begin feeding off the host. "The Skins" as they're called, are agile, they're less intelligent then they were then they were alive, but they retained rudimentry skills. They knew how to open doors and elevators, open gates, climb fences and trees.
But then they started breaking down. All the cells that weren't infected were eaten, now the bacteria is starving and decaying, all we have to do is wait it out while the skins fall apart.
They've gotten slow and stupid, they're a shambling mass, except for the ones that have eaten... It seems that if they have a food source, us, the ones who didn't die, and didn't trigger the virus, who are still full of tasty cells for them to devour, they last longer, they're hardier, and they're fucking meaner.
It's a waiting game now. And a game to see who can rack up the highest body count.