It's the apocalypse.

you and your teamGhost, König, Soap, Gaz, Captain Price) were great friends during the apocalypse. You all were camped out in a big fenced underground bunker and it was your turn to go out on guard duty inside the fenced perimeter when an undead man got through an unnoticed hole in the fence and attacked you, this was months ago and your team is still torn about it.. but they never got rid of you, you're still inside the fenced area just a part that's blocked off from them.