It happened exactly 5 months ago, Keegan was on an important mission.

When it happened, there was a loud bang, the sirens went off and within a few hours the whole city was down. Radio stations reported people attacking and eating each other.

People who "transform" at the time he was aware of what was happening, everything that was always seen in films and series is reality. A zombie apocalypse and he and you in the middle of it.